Maybe the systems bios is getting corrupt?? (battery also have connection with cmos) I dont know how long CMOS lives, but what i know is, that after 23 years later it could give you problems with the cmos chip...
What the best is to do? These computerparts we are using are very old.. So backup every cmos bios rom you have should be very smart to do.
So if something gets wrong you can always write a new cmos rom chip..and should stay save...

I know the most of the boards i have here about 40% have a bad cmos(or is something wrong with it), or is never upgraded to the final version..

Thank you for the reply!

I am actually not sure how I would go about backing up the cmos bios...I did not even realize you could do such a thing.
I will have to search these boards or google it.

I was also wondering if maybe it had something to do with the power unit itself? If maybe it had lost some juice or something?
I don't know, I am reaching..my gut tells me its something very internal on the motherboard that I don't possess the knowledge to fix..

🙁

My advice:

• Remove all socketed chips, clean the legs and sockets by lightly rubbing them with the tip of a jeweler's screwdriver, then put them back (careful not to put them backwards).
• Scrub the memory and expansion slots with a clean toothbrush dipped in isopropyl alcohol and clean your expansion cards' edge contacts too.
• If nothing works, power up the board in a bare test bench and start flexing it around and pushing down on the chips, see if you can make the glitched patterns change or disappear.

this is no BIOS problem... 133mhz's advice on cleaning the chips is good and you must also check for leaking/bulging capacitors or any strange fluids on the mobo. Since this happens even with another vga the best guess is that some part starts failing and screws timings or something. It wouldn't surprise me if a recap fixed the problem.

Also, if possible, check the 386 chip if it's overheating. And also change the PSU if possible.

I would also check the PSU (open it up if possible) for dust build-up. This might sound strange, but I just read somewhere that PS/1s and PS/2s should be brought to room temperature before turning on...don't know why. Do you have your PS/1 in a garage or basement that may get colder than usual by any chance? Just throwing it out there.

Thank you all for the replies.

The computer is in my living room which can fluctuate the temperature a little bit but never colder than 66 and never warmer than 74 degrees. But being older it very well may not
take much for it to be affected.
It definitely sounds like a plan to give the motherboard and PSU a really good going over, making sure everything is clean. For being over 25 years old, the inside of the unit is remarkably
clean. The previous owner was apparently a church.
Some kind of overheating still sounds plausible since it doesn't start happening until its been running a handful of minutes, and then the only fix is to let it sit for a little while...

Thanks again everybody!