Yeah I doubt it's firmware, especially now that I know this isn't a June 2004 G5... confirmed by a "©2005" stamp on the logic board. Thanks for enlightening me to the existence of another 7,3!
Not sure when I'll get to making a new post, as I'll do some more troubleshooting first, so here's the short of it: it's crashing a lot, seemingly randomly. Sometimes it's an infinitely spinning beach-ball, other times multilingual screen of death, other times just freezing up. It's even crashed on the first boot right after a clean install of 10.5.4 on a new hard drive.
However it passes every test I've put it through, including multiple runs of AHT. The memory is all Apple-original and has passed AHT, as well as Memtest booted into single user mode. No peripherals or PCI cards. Sometimes it won't crash for days and other times it won't boot at all.
The hard drives are properly jumped to run in 1.5GB mode and I'm running low on ideas, other than the possibility that this is a problem with one of the procs or the logic board. See... now I've gone and written more than I should, but I suppose I'm just hijacking my own thread! 🙂