Thanks Syntho! I tried simplifying the hardware configuration to the bare minimum usable, but to no avail. This is after I'd already done my best to trim to a minimal extension set (and had freezes/errors with extensions off). My 9600 didn't come with the original processor card, so more and more I suspected the G3 upgrade was the source of my problems.
Because
my 9600/200 would have a 50MHz system bus with the stock processor card, I turned on dip switch 2 on the G3/300 card to use a 50MHz system bus
per Siber-Sonic's findings. It worked, but it wasn't the solution to my stability problems. Still, I'm
guessing this is the best mode for any 50MHz bus 604e system with this G3 upgrade, and similarly you might want 1,2,4 for 50MHz bus Kansas systems.
The Solution: I finally made some progress after digging into
the G3 upgrade issues on The 6400 Zone, specifically the parts about Speculative Access.
Newer Technology claimed they had solved the these problems in hardware on their MAXpowr G3 so a software fix wouldn't be necessary, but I don't believe it anymore. I found an extension called
ROM Fixer (
download) that disables G3 Speculative Access, and it seems to have solved my stability issues.
Among other things, MechWarrior 2 was giving me a consistent, repeatable crash at the pilot menu when Speculative Access was "Enabled (I/O Compatible)", but now it works!
It looks like my only remaining problems are with files that already got corrupted (especially application resource forks) so now I'm doing fresh software installs. Aside from that, it can get stuck at the grey screen with cursor when it starts booting, but the Apple three-finger salute usually gets it moving. I assume this is another oddity of the G3 upgrade.
Hopefully someone else struggling with a G3-upgraded Mac will find this information useful!
All I've got on mine is a Sonnet PCI card for an SSD (which is actually for a recording drive, not the OS), a SCSI adapter and faster/quieter SCSI HD as the main drive (having a SCSI drive as the OS drive is important)
I'm using the
AztecMonster II to boot from a SATA SSD on the SCSI bus, which is working great. (The stability problems were happening before I did this upgrade.)