Ditto...I've have an 8600 that was giving me errors ohmygod I was ready to rip my hair out after several days of nothing but failures of OS installs...
Anyway, I *thought* it was the SCSI bus and started using an external SCSI drive, and things seemed better for a little while, but then the same sh*t started again...
Then by accident tooling around in there, my hand brushed the heatsink, and OUCH! Whoa...it should be warm/hot, but not blistering to the touch...Anyway, I got some advice on to how to but thermal paste on the chip, cleaned it off and applied new paste, and yes, the thing started working well again.
That never would have been my first guess, or even my 30th, but now that it has happened to me (very much like your situation), I am more savvy to it when things really appear to be 100% weird and inconsistent.