Perhaps the IDE port is broken? Have you tried a different port/PCI IDE card?

Ooh good call - I’ve got a few PCI/ISA I/o controllers to try

Update:
Turns out the I/O Controllers I have require the BIOS/Mobo have the onboard controller disabled, which I can't do at this time.
I managed to create a boot disk version of the BIOS -- and it boots to the menus but as soon as I pick something (like test -- fails after checking 8-9 areas sometimes graphics and sometimes onboard ports)
I created a BIOS update disk, but it fails at "gathering info about your system"

I've got other things to do so I've decided to set this machine aside and have a think on it.

Next step will be to figure out what step happens after this moment in a boot sequence of an operating system.
Either after a CDROM driver loads, during a full system check etc. My guess is something to do with a controller, a PnP system.

Clearing the BIOS, pulling the battery, different boot mediums (HDD, Floppy & CF) different video, sound and no riser at all... I'll also have to re-look at any possible traces near cache or see if I can find an external cache to plug into it.
It does have a certain amount of onboard soldered cache so you'd think the module header wouldn't be needed.

I suspect the motherboard could be warped and you have gremlins in your system due to this. I've experienced similar issues you're describing due to this problem. Do you see any difference if you put the system in any other orientation, for instance placing it in vertical position compared to horizontal? Heck, even try upside down if everything is properly secured.

The motherboard that warped in my case was the ASUS P65UP5 which has poor supports to the case compared to other AT boards (and they are even worse than ATX). I pulled the motherboard out of the case and glued some supports underneath on the case (non-conductive) so when the hardware was installed on the board, it would stay as level as possible.

@Vetz - This desktop board is pretty well secure - although I've had the whole board completely out of the machine to replace the caps as one looked suspect (there's only 3 electrolytic on the mobo itself).
The whole thing seemed pretty straight... there were a few suspect traces (weird too on a board of this era because there's no corrosion anywhere on it) but they don't appear to be an issue. So hard to test on a board like these because everything's tiny. In some cases the components are smaller than those on P4's I've repaired.

I think your right though, there's definitely something up with this motherboard... but I *hate* throwing out stuff older than P4 even if it means I'll have to practice my SMD skills. I've repaired a VLB Diamond Viper CPU that fell off and a few others like it... it's always worth it if something's going to be enjoyed by the next generation.

ThisOldTech wrote on 2021-05-18, 00:29:

@Vetz - This desktop board is pretty well secure - although I've had the whole board completely out of the machine to replace the caps as one looked suspect (there's only 3 electrolytic on the mobo itself).
The whole thing seemed pretty straight... there were a few suspect traces (weird too on a board of this era because there's no corrosion anywhere on it) but they don't appear to be an issue. So hard to test on a board like these because everything's tiny. In some cases the components are smaller than those on P4's I've repaired.

I think your right though, there's definitely something up with this motherboard... but I *hate* throwing out stuff older than P4 even if it means I'll have to practice my SMD skills. I've repaired a VLB Diamond Viper CPU that fell off and a few others like it... it's always worth it if something's going to be enjoyed by the next generation.

When I had my board out for inspection it was straight and I could not see any visual damage, still the computer only worked as it should when it was in horizontal position, not in vertical. I'd give it a shot to try another orientation or add some pressure on a specific part when booting up.