Borderline power supply which got so borderline that now the HDD load spikes manifest as problems in other locations?

Interesting, hadn't thought of that. Will try other PSU.

PSU is fine, tried a second known good one. This is getting obnoxious. With all the problems I'm having, I'm just about ready to ditch the 286 in favor of a 386.

Sounds like you're getting excess noise in the serial bus. Could be a filter capacitor gone bad. I would check out the IDE card and Serial card first. Also, remove all unnecessary cards that aren't used to test.

put an oscilloscope/logic probe on serial line and sniff where its coming from

This is not on-topic per se, but is there a specific reason that you run a soundcard from 1994 together with a CPU from ~1987?
I would guess that whenever there is some soundblaster operation in progress that it severly hits the performance of the poor 12Mhz 286.

I once tried a 1992 SB16 model CT1740 in a 386-16Mhz and even that was severely bottlenecking.

jaZz_KCS wrote:

... that was severely bottlenecking.

Sorry for OT again, but exactly this is the reason why in my current retro project (upgrading my IBM 5170) I am going to use PCI sound cards.
Long ago I read a report in the German "c't" which analyzed the ISA soundcards' impact on computer performance and found out that their 4.77MHz ISA DMA can slow down PCs up to 30%. Imagine, just for playback of a stereo track... I do no longer want this.

jaZz_KCS wrote:

This is not on-topic per se, but is there a specific reason that you run a soundcard from 1994 together with a CPU from ~1987?
I would guess that whenever there is some soundblaster operation in progress that it severly hits the performance of the poor 12Mhz 286.

I once tried a 1992 SB16 model CT1740 in a 386-16Mhz and even that was severely bottlenecking.

It's the only sound card in my collection I could get to work with the 286 machine. I have 2 PAS16 cards, and a Covox Sound Master II, and none of them worked with the 286. The Covox Adlib works, but the sound blaster emulation drivers require a 386.

Despite the fact that the soundblaster is so new compared to the 286, it works flawlessly! (besides a driver disagreement between it and the SCSI card I am trying to get working) No problems whatsoever. I just use the drivers for the Soundblaster 16 for most games and it works just fine. I would prefer to have an older SB16 or perhaps a SB2.0, but I am too cheap for that, 🤣.

I ended up ordering a 386 motherboard to upgrade to, as dealing with 286 specific problems is getting kind of old. I just remembered I do have another IDE card, so I will try it next time I feel like messing with the machine. I probably won't do too much with it until I get the motherboard, though, because I don't see the point in spending too much effort on the 286 when it is ultimately going to get replaced. I am probably going to build the 286 machine back with some more appropriate hardware when I get some, though.