I'm going after my 7300/G3-275 right now with similar issues. I discovered I had the hard drive SCSI termination wrong... that made a huge difference. Just about to clean/regrease the CPU heat sink. We shall see.
So does anyone know of a good free memory test/benchmark/burnin utility for Classic Mac OS? Tons for Winders boxes but I can't find one to exercise my 7300.

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Frakkin box. It's taken up too much of my time. I really had my hopes up there for awhile. I actually had OS 8.5 installed and the unit would boot every time. Upgraded to 9.1 - OK. Try installing OS X with XPostFacto - crasho. Then the machine won't even boot until I strip it right down to minimal RAM, take all the cards out, etc. Then it'll boot. I can then slowly upgrade it back to its original self and it'll work for awhile. Then it'll die and I'll have to repeat the same thing all over again.

It's posted in freecycle RFN. Good riddance to it.