I've tried replacing power supplies (I've got two spares), memory (and, in the process, tripling the memory in it from 128 to 384 MB), disk drives, disk controllers, video cards, and Ethernet cards. I've also tried cleaning the contacts on the CPU board's edge connector with a pencil eraser (it's been years since I had to do THAT), and blowing accumulated dust out of it. I've also jumpered the power interlock so the thing will run with the cover off.
The next step is to replace the CPU board. I've got a 500 MHz processor coming off eBay. After that, the only thing left to replace, aside from some wiring harnesses, is the motherboard - and, so far, I haven't found one of those. This is getting irritating.