might be power supply on last legs, might be crushed pin in one of the slots where you were reinstalling cards
measure voltages and carefully look into ISA slots with good light

Thanks for your answer, I had a careful look on the isa slots, they looks in perfect condition, and as I tested several of them I don't think this could be the issue. I have a post card and each voltage indicator leds are turned on, but I'll try with a multimeter to be 100% sure.

Does sound ike it could be PS - be aware that "voltage" isn't the whole story.
There's also noise, caps going bad could lead to excessive noise while average voltage looks OK.

There's also the possibility of drop-outs when "high power" cards try to start, when debugging issues like this I always start with on-board video if avaiable, or a a super-simple/low-power card (like ATI VGA wonder) if not.

If you have access to a scope, watch the power rails as the system tries to start. Everythuing should be rock solid.

Also, since you've been swapping things in/out ... have you tried "factory reset" - harder with no video, but disconnect BIOS battery for a good long time - also depending on mainboard there might be a jumper.

mobo and cpu models would also be helpful

vbug wrote on Today, 13:05:

Thanks for your answer, I had a careful look on the isa slots, they looks in perfect condition, and as I tested several of them I don't think this could be the issue. I have a post card and each voltage indicator leds are turned on, but I'll try with a multimeter to be 100% sure.

It would be enough for any two opposite pins in any ISA slot to touch to mess all other slots. If the card isn't recognized in one ISA slot, it is very expected it won't be recognized by others, unless the issue is a severed trace.. but that is unexpected in your case, as traces don't suddenly degrade and get severed by normal use.

My bet is on one of the caps giving up finally. Cap failiures have this symptom of gradual degradation until final death. A quick continuity test across the caps might be useful.

vbug wrote on Today, 11:31:

(AMI bios, 2 short (high), 8 long (low))

2 short, 8 long? that doesn't seem to be a standard ami beep code .
I guess you don't have a post card you can stick in the machine to see which post code it get stuck on ?

Either way, I'm gonna advice you to take everything out of the case, all cards & mainboard, memory, and hook it up outside of the case one by one.
Verify the behaviour of each step, start with the mainboard without memory hooked up to the psu,
add the memory,
add the keyboard,
add the video card with monitor attached.
verify beeps & led behaviour at each step.

So I tested the voltages when turning the computer on, it seems okay, stable voltage for both 5V and 12V. I have a postcard, when the motherboard starts to beep the post code is 1-2F for 1-2sec, then it moves to 1-3B and stick to that code. I already tested to turns it on with only CPU, RAM and basic ISA video card, same result. About the caps that could be dying, do you mean on the PSU, or other caps on the motherboard ?

I tested the DALLAS battery and as it was a bit low I replaced it (so I removed it and wait for some minutes so I assume the bios has been reset). Now it stops on code 3B meaning it waits for the user to press del key (probably because the bios has been reset). But still no display

I join a PDF presenting the motherboard model, and some photos of the ISA/VLB ports

here is a video of the boot sequence (as zip file because video files are not supported as attachment !

Have you tried removing the ram and reseating it, or if possible using different memory modules?