This will be a long(er) post.
I have this 486-66MHz PC in my Retro PC Gallery.

It is using an undocumented and rare mainboard:
https://theretroweb.com/motherboards/s/matra-486-vip

It's unstable and I want to fix is, not change it/throw out.

Issues:
1) The DOS crash:
(Soft?)Freeze in DOS. All looks fine, I want to start Norton from DOS, type "NC" ->hit Enter -> Crash.
(Soft?)Freeze = Enter a command in DOS and PC freezes command not executed, DOS cursor still blinking. Responding to NumLock key with huge delay. Press NumLock... 30-60 seconds or more and Numlock led is ON. If I press a bunch of letters the PC Speaker gives an alert not to molest further and freezes for good, no more interaction between us.
Ctrl+Alt+Del not responding.

2) The Windows crash:
BSOD in Win95. All sorts of other errors in Windows95 like registry corruption and explorer application crash with 2 buttons (Ignore or Restart)...

3) The Funny crash:
Memtest+ runs for hours and no issues.
"Pass complete, no errors, press ESC to Exit"
I press ESC, it says:
Testing: "132K - 16MB Halting...".
Little red "+" sign next to Memtest86 is blinking for hours; the little shit crashed. 😁

Things I've done to it are unimaginable. Changed and crosschecked:
- L2 cache (on/off in BIOS)
- L2 cache (removed from mainboard)
- Different Cache & TAG chips ranging from 128K till 512K
- CPU
- Memory modules (2x4 60ns and 2x4 70ns)
- PSU
- Video card
- Running bear minimum: One SIMM (4MB), no CF (HDD), no cache, one VGA.
- HDD controller
- HDD type (LBA vs. CHS)
- Power Management disabled in BIOS.
- TB SW does nothing observable.
- There is a "Green" mark silkscreened near the TB button and TB led. Does nothing.
Update:
- Tantalum caps are not shorted or in an exploded state.
- Removed the "modern" PC Speaker.
- Tried 2 AT keyboards.
- Tried basically all BIOS settings.
- Memory Slots 0-1 and 2-3.
- Bus speed 33 and 25. (Currently set to 25)
- Tried 3 KBC chips, all working. System still crashes the same way.
- Solder joints (individually probed) for main chips.
- Program load/exit loop below -> freeze.
I could only stop the loop with Ctrl+Ald+Del or Reset button. Based on the count size system did not crash. Keyboard crashed.
"
@echo off
:top
command /c exit
echo x >> count.txt
goto top
"

- Append-only loop below -> freeze.
I could only stop the loop with Ctrl+Ald+Del or Reset button. Based on the count size system did not crash. Keyboard crashed.
"
@echo off
:top
echo x >> count.txt
goto top
"

Pure CPU loop below -> freeze.
I could only stop the loop with Ctrl+Ald+Del or Reset button. No count.txt I guess system did not crash. Keyboard crashed.
"
@echo off
:top
goto top
"

Observations:
- Most of the crashes are random and instant and sometimes it happens after a keyboard input during an idle period.
- Windows idles for hours and no crash. Open My Computer->Explorer app crash or a BSOD.
- Would not crash during benchmark running. Running for over 12 hours.
- Would not crash during game running. Games run for hours in a loop and no crash.
- Memtest+ runs for hours and no issues.
- Power Management disabled in BIOS. However! I have never seen such a PM on a 486. If Power Management is enabled it turns off the CPU and video even if benchmark or a game running and there is no keyboard input. Never seen Doom with 1-2FPS on a 486 right before video is cut(going to sleep). After a keystroke all comes back... sometimes fine sometimes with a crash.

===================================
Observation update 1:
I think I narrowed down the issue to something keyboard related.
- Boot with keyboard connected.
- NumLock LED lights up.
- Boots and Memtest+ started.
- Press "C" for configuration, works fine. Also, I can navigate in this menu system.
- Pressing "Space" activates the Memtest+ scroll lock.
Now:
- Pressing NumLock button does not turn off the NumLock LED. It has no effect.
- Pressing "Esc" freezes the PC.
- Restart. Boots and Memtest+ starts again.
- Pressing "Ctrl+Alt+Del" ... well, I loose the keyboard but the Memtest+ continues to run fine, no freeze.
- Re-connect the keyboard, acts as nothing happened. I can press "C" I can navigate in the menu system, I can press "Space", so on.
- Pressing "Ctrl+Alt+Del" disconnects the keyboard, again. 😁 Pressing "Esc" freezes the PC.
===================================
===================================
Observation update 2:
One more thing I noticed during memtest+.
I press NumLock button and LED not turning OFF (stays ON).
However there is a delay here that I can "play with."
Maybe someone experienced similar and can figure out what is happening here.

I press NumLock button and then press "c" for configuration menu -> the menu pops up in an instant.
I press NumLock button quickly 5 times and press "c" for configuration menu -> the menu pops up with a small delay. (~7-10 sec)
I press NumLock button quickly 10 times and press "c" for configuration menu -> the menu pops up with a BIG delay. (20+ sec)
I press NumLock button quickly ~20 times and press "c" for configuration menu -> the menu pops up with a delay almost reaching 1 minute! Huge delay.

When pressing NumLock button in a quick succession there could be a small (NumLock) LED dimming once or twice. Barely noticeable, in the millisecond range. - not sure how relevant this could be.
===================================

Since the mainboard has no documentation it's like jumpering in the dark with eyes closed.

Jumper findings:
-Besides the BUS I could not change/jumper anything CPU wise: multiplier, CPU type, so on.
CPU1: 80486 DX-2 66MHz - works fine at 66MHz.
CPU2: AMD Enhanced Am486 DX-2 66 MHz (Am486DX2-66V16BGC) - works at 100MHz (with a 33MHz BUS) and 75MHz (with a 25MHz BUS), for some reason it's using an 3x multiplier. This one has the 5x85 core, I presume that is the reason for this behaviour.
- JP2 = FSB select (25/33/40 MHz) — silkscreened (the only thing).
- JP15 = Tag pin 3 shorted to cache pin 2 (cache enable?) - this must be closed at all times when cache chips are installed.
- JP16 = Single/dual rank L2 cache. 1-2 dual. 2-3 single.
- JP82 = 512 KB cache size select.
- JP24 = This one is strange, not seen before on any other mainboard. Roughly halves bandwidth. L1 is fine, L2 and memory gets -50%. It was the only setting that enabled BANK0 only L2 cache to work, however with huge penalty to both L2 and memory. 2-3 - Normal; 1-2 - Reduced bandwidth (-50%).

This board has major L2 Cache issues:
When only BANK0 cache is used, BIOS/POST shows the right amount of cache.
CACHECHK or SPEEDSYS shows no L2 Cache. After L1 it's memory speed.
Whatever I tried with BIOS (timings and everything) or jumpers it would not work.
Using BANK0 and BANK1 for a total of 256K cache.
Currently all cache chips are removed, crashing still occurs.

I welcome any ideas on what to check where to look: except the trash bin. (for now)

Regards,
Attila