The reboot issues def sounds like a power problem or maybe a memory issue.
You've replaced the PSU with a known good/new baby AT psu?

Have you tried replacing the memory?

Check for cold solder joints on the ISA connector. I'd also check for tarnish/ISA edge finger damage on the prizm card.

IF all of the above; maybe your CPU/mobo is toast because the psu fan stopped working and "overvolted" the combo?

Maybe you should send the PC off to Borygard to fix?

It could be anything on the mobo going bad.
But you really need to test each one of your components long enough in a good environment to ensure they don't have the "warm" reset.
Unfortunatly; if you have id-ed the mobo as bad... you maybe sol.
Borygard may be able to fix it... but mobos generally are hard to work on. This one maybe old enough that he can do some fixes... but a modern one is a toss and replace.