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?