Sounds like a memory problem to me -
What speed is your RAM rated for, and what are you using it at?
Even if it's rated 60ns, you might try it at 70ns if you aren't
already and see if the crashes stop.
You may also need to change your precharge settings to the slower
option (I forget which that is - check your PPC documenation).
I have a similiar problem though, but only with certain games
like Napalm and QuakeWOS - they always crash within a few minutes.
Lowering the memory speeds didn't work for me sadly.
