You can buy RAM from a company such as ramjet.com and take out the apple ram to see if that makes things better. At the very least you'll have more ram for when you fix the problem
If it does solve the problem, try and get apple to give you new ram (but don't leave the 3rd party ram installed or they will remove it, say that was the cause of the problem, and not fix anything). Tell them you pulled RAM from another powermac g5 and that it fixed your problem
You can also try running
Memtest for OS X.
This app seems to provide a graphical front end to memtest but I don't think it's as up to date as the other program.
If you have other devices connected to your machine (USB devices, PCI cards), they might be causing issues as well.
If you installed quark 6 into your applications folder and never repaired permissions after, that could also be causing you problems.
