Use the 25PIN serial port. That is what I did.

Oh is it one of those standard two-button PS/2 mice as pictured here?

I had one a long time ago which exhibited the same odd chaotic cursor jumping. Unfortunately I got mad and threw it out as a young brat, but I bet it was an issue with some capacitor inside.

Try a different PS/2 mouse to rule it out.

I have some older IBM mice that had problems with giving keyboard errors on boot and other weirdness. Ended up being the single capacitor inside there.

Thank you for all the replies.

I have tried several different mice, most of the two button variety from various companies. No luck.
Its frustrating because it appears to work fine for a few seconds and then it starts jumping and clicking randomly.

I was able to grab a PS/2 to 25 pin serial adapter from Ebay so I can see if plugging it into a different
port helps any.
I will also do a close look at the board to see if any capacitors look funky, I hadn't thought of that...
Any other ideas would be greatly appreciated. I like this computer a lot otherwise!

Thanks again

Is it possible there is an IRQ conflict or even your keyboard? If you can start up without the keyboard and set the bios to disable keyboard warnings have your autoexec auto load your mouse driver and a program (like edit.com) and see if it works. Sometimes if the keyboard is bad it can goof up the ps/2 mouse or vice-versa.

Since it 's happening on multiple mice I assume you haven't tested these mice on another computer with known working ports? I could see one or two bad mice but if it's every single one then yeah something with that port (or possible resource conflict) or like i mentioned the keyboard could be the problem.