Keyboard and Mouse Stop Working After A Few Seconds in POST/BIOS

[Jentralith]

Shortly after booting up the computer and entering the BIOS the keyboard stops working.
I've tried plugging in a mouse using one of the the onboard serial/com ports and it also stops working.

Another issue is that the BIOS seems to fail to load when more than just the basics are plugged in.
With just a cpu ram and video card I can get in to the BIOS and have a few seconds 20-30 or so, before the keyboard stops working.
With cpu ram video card and mouse I only get about 10-15 or so and with a floppy connected or the soundcard plugged in the BIOS won't even load.

While attempting to fix it I cleared CMOS. When the pc booted the press F1 to enter setup message came up but the keyboard stopped working before I could click.
I did however notice that the cursor was still flashing so it doesn't seem like the computer crashed. That's why I think it's the keyboard and mouse not working.
But the BIOS not loading makes me think that it might be something else.

This also only occurs in the POST/BIOS. I've used a boot disk to get to a prompt and can the use the pc without it crashing.
I've successfully loaded simple dos games that can run from disk and played them without any issue.
I can't do anything major however as I can't get a hard drive to work, but that's probably due to a BIOS setting which I can't change before the keyboard/mouse dies.

I've tried different keyboards and mice with no change. I tried pulling and reinserting the clock battery to clear and weird settings. I used a jumper to clear CMOS.

I tried using an ISA video card (CIRRUS LOGIC PT-521/3 ISA GPU) but it was finicky. Not sure why. Sometimes it would work without issues, others I'd pop it in and get no display.
Other times it would display some text with a cursor but that's all (no POST displayed)
The card works perfectly in a different computer so it's something to do with this motherboard. Could be bad contacts in the ISA slots but I tried it in different ones with no improvement.


There does not appear to be any damage to the motherboard, no burn marks or loose connections. Except for the bottom ISA slot which has bent contacts.

M-Tech (MTI)/Rise R418 PCI-486 Rev 1 - This is a picture of mine (https://imgbb.com/r65QQZB) (It shows the EDO ram in which is currently removed and not heatsink but one is installed when using the system.
AMD 5x86 - With Heatsink and fan and lowered to 100mhz with jumper settings.
AMD Mach 64
Sound Blaster 16 (Removed during all testing)
PSU is a Corsair CX450M using adapter (I've tried it with a working older PSU with no change)
Ram is nec - 424400-70 9115FUO29 (I use 8 sticks but have tried with just 4 with no change) I also have EDO ram but it's not currently installed.

Any help would be appreciated.

 

[modem7]

I did however notice that the cursor was still flashing so it doesn't seem like the computer crashed. That's why I think it's the keyboard and mouse not working.

The flashing cursor is done by the video card. As long as adequate power is getting to the video card, the card will continue to flash the cursor until configured not to.

 

[modem7]

PSU is a Corsair CX450M using adapter (I've tried it with a working older PSU with no change)

Months ago, I had a motherboard out-on-the-table, with power supply and keyboard, etc. Known good MDA/CGA video cards sometimes showed no display or a corrupted display. Intermittent RAM failure. Everything else appeared to behave itself. The next day, I noticed that the video cards became more stable the closer they were (slot wise) to the motherboard's power connectors. I discovered that the motherboard's +5V rail measured about +4.6V. Problem fixed by removing then refitting the ATX-to-AT power adapter cable that was between the power supply and the motherboard.

 

[Jentralith]

I reconnected the at adapter and checked that voltages.....stable 5v on all the red wires.