Will the system run fine for a few days if only one soundcard is installed? Maybe a IRQ sharing conflict or even dos emulation of the MX card?

Tested memory with memtest?

Lastly could be a hardware issue like bad capacitors on the motherboard? I have a pile of socket 462 boards with faulty caps to fix.

Does Prime95 run in safe mode?

Yeah it could be a PCI thing. It's strange that it didn't happen right away though. I'd try moving the cards to different slots. Some PCI cards really do not want to share their IRQ with some other cards. Rather annoying to say the least.

I couldn't get either an MX300 or a Montego II (both AU8830 cards) to work properly in my KX133 Slot A board... and it did seem to largely revolve around the SB Pro emulation of the Vortex. You might try disabling that particular function in device manager, but what finally fixed the problem for me was just swapping in a SB Live.

When I just had the mx300, the system was trouble-free. Or at least it never failed at booting. So this problem is something to do with having both the yamaha and mx300 cards.

I probably should do more systematic testing to get to point where I can reproduce the problem, though I have noticed lockups don't always seem to happen at exactly the same point.

Really reminds me of a pa-2013 that I wasted countless hours on trying to solve random freezes. Worst board ever imho. Had hoped such days were over. 🤣 But at least I know I can roll back to what seemed a stable system here.

Well, it could still be less directly due to the SBPro emulation, since I believe the Yamaha card has its own form of SB emulation... if you have two different cards trying to emulate the same piece of hardware, it'd make sense that there could be some trouble as a result.

I also see that I have onboard audio, and that has SB Pro emulation too 🤣.

Weird, I have this device shown as PCI Multimedia Audio device, which I've disabled, and this must I'm sure be the on-board sound. Which is disabled in the bios .

Anyhow, I also disabled the one serial port I had previously left enabled [parallel is already disabled] and that may have solved the problem. At least now when I shutdown and then reboot the system I can change which hardware config I use and I don't get a freeze before logging in. Haven't tested beyond that yet.

The MX300 is now using irq4 for it's Multifunction Parent, which I thought it what the serial port had been using, so maybe there is some explanation there.

In each of these hardware configurations only one of the sound cards has the legacy emulation enabled [though I left the MPU401 on the mx300 enabled under the "Yamaha-all-enabled" configuration].