It shows Pentium III MMX on the bios screen - surely that should say SSE.
What does it show for the CPU with the 800MHz P3 in there - could it be a microcode / CPU stepping problem?
Or maybe it's a voltage problem, the detected CPU voltage is lower than the rated 1.7v.

The microcodes are the last version, voltage shouldn't be an issue since it's underclocked, but I raised it on the bios, now I get 1.76 but the issue is still there, I also tried an older cpu, it always get detected as pentium III mmx, but sse got detected on windows.
I'm thinking to try another psu.

Is that a branded or a generic slotket? Also, might be nothing, but MSI seem to have removed that cpu from the support list at some point

Assuming that everything was stable with the P3 800 running at the same FSB frequency on the same slotket, it does point towards the CPU itself .
I have seen CPUs with bad cache exhibit that sort of behaviour . You might want to try with L2 cache disabled as a test (do not disable L1, it will be unbearably slow) .

Thanks for all the answers.
I suppose is a generic sloket, but it has many jumpers so it shouldn't be that cheap, I have another one with no jumpers at all that is called: D370A VER:1.0
I'm pretty sure the system was very stable, I always used it as testing bench and I never had issues other than having to clean install windows 98 from time to time.

I may have found the issue, I tried 2 ati cards, to be precise a Radeon 9200 DDR VIVO and a cheap Radeon 9200LE, I swapped the cpu to a slot1 celeron 400, but windows 98se hanged during the boot for several time, in the end I swapped to a nvidia card and everything went smooth again, now i'm with the P3 1000 at 750 everything goes fine both 3dmark99 and 2k runs smoothly, does everyone know about any compatibility issue between the bx chipset and radeon 9x00? I downloaded the drivers from amd website...

Edit: With the gf4mx440se works at 1000mhz with 133 fsb.

Yep, some cards draw too much power over the AGP bus. I just had to swap out an Nvidia FX5200 from my build for that reason.

Put an old 128 Pro in there and it runs fine.

That's on a Gigabyte GA-6BXE build with a P3 700.

texterted wrote on 2020-04-10, 23:16:

Yep, some cards draw too much power over the AGP bus. I just had to swap out an Nvidia FX5200 from my build for that reason.

Put an old 128 Pro in there and it runs fine.

That's on a Gigabyte GA-6BXE build with a P3 700.

That would be an explanation but wouldn't be a radeon 128 pro too old for the system?
I tried the gf4 ti4200 and it works fine too!

Well, I don't need to do too much with it and it handles basic games quite well. I have another 98 system for games.

Those Ti cards draw plenty of power too, keep an eye (or a finger) on the power regs on the MB!