I'd start by cleaning the contacts of all the memory and ISA cards. Use a pencil eraser to clean them.

Exercise the ISA/VLB slots by inserting and removing cards multiple times to try and break any corrosion there.

I'm not too sure what could be causing your memmaker problems though. Have you tried a different power supply?

Edit: Try popping out the keyboard controller IC and pushing it back in if it is in a socket.

I also have issues with a PC Chips M919 and serial mouse. Thought it could be that it was mismatched with the serial port connectors, but when I tried it with serial and InPort cards there was still no joy getting the mouse working. Shame as it's otherwise an OK board, but I'd put your troubles down to PC Chips flakiness 😀

So I cleaned and reseated all of the expansion and RAM cards/slots. However, it turned out to be a bad hard drive! I am not sure why everything worked when I booted the drive in the P3 system. Also the diagnostics didn't find anything wrong with the drive. But when I swapped the drive for a similarly old 504MB Connor everything started working! I guess the PC Chips motherboard is just more sensitive to failing drives?

The serial mouse still doesn't work, but from JidaiGeki's reply it seems these motherboards are not exactly mouse friendly. 😊

For serial, did the serial bracket come with the motherboard ? There are two types of pin arrangements for serial headers, and to make this more fun, there is no way to visually differentiate them :p
You can open the serial connector and look how the thing is wired though. It can either be two perfect rows of soldered wires or staggered wires.

With older systems and the age of the parts, there's a lot to go wrong, but quite often it ends up being something simple. I had some strange behaviour on the weekend from my Penium 166 system. I got Windows 95 installed, but soon after setup finished, and after the first boot, Windows wanted to recover the registry. That... isn't a good sign... so I ran Memtest86+ and ran 2 passes on the 32mb EDO...but it found no errors. So what gives? Bad 95 CD, bad CD Drive, bad IDE cable... I was using a new 40GB IDE drive, and had installed Win 95B OSR 2.0 with the Fat32 support, so I didn't at first think that'd be an issue. It had no problems partitioning and formatting, then installing onto the 40gb... until the registry kept corrupting. I ended up dropping the partition down to 3x 2GB Partitions, installed 95 onto the first one, and all has been happy since. But with older system, expect the odd strange thing to happen once in a while, if for nothing else but the age of the parts. If you're using an older PSU that from age is outputting flaky voltage, you can experience for much odd behaviour, you'll think the system is possessed.

With older systems, you've always got to make sure connectors/PCI/AGP/ISA cards and contacts are seated properly, over time older parts warp through heat and age. With old PC builds, two things I won't go old on is a PSU and Hard Drive. Keep those new, or as close as possible. I love going 100% vintage, but using oldwe PSU's has usually given me nothing but grief. As for Hard drives, it's hard to find >10GB one's that don't whine... so I just use larger and newer IDE drives, or CF cards. Eliminate the most common parts to give you trouble through age, and you'll enjoy the system a lot longer without issue. (From my experience)

Sounds like ram a parity issue. Get checkit and run the ram test in Dos.