In a previous thread, I mentioned the problems I encountered with two recently acquired Power Macs (Dead Acquisitions: 8200, 9500 -- http://www.applefritter.com/node/6474). After further testing, I've concluded that both the 8200 and 9500 boards are dead, as is the 9500 psu, but the 8200 psu seems sound.
The third Mac in this bundle is a 7100 and, like the other two, it had been stored in a damp garage. I left it for a week or so to acclimatise indoors and cleaned the obvious filth off the motherboard, case and internals. It was surprisingly interesting for a 7100: lots of RAM, a 2GB disk, unusual Formac graphics card, ISDN card.
The 7100 worked fine for a couple of hours so I wiped the disk in preparation to install a clean operating system. During the reinstallation, I encountered system freezes and bus errors. So I wiped the disk again, assuming that the Mac OS installer didn't get on with the disk formatting tools and tried different system versions etc.
Then the power supply went bang. Literally. That explains the system freezes...
The moral of this story is that old computers don't like being stored in damp sheds and attics. Look after your old Macs and protect them whenever you put them in storage. When you acquire Macs that have been stored badly, handle them with caution and don't be surprised when things don't work.