G5 1.6 freezes with certain applications

I bought a used G5 1.6 about 6 months ago and am struggling to find a solution to my problem - the computer consistently freezes when using certain programs. The biggest culprits are browsers (Safari, Firefox, Opera all create this situation, with Plug-Ins activated and without), but it also happens when using Excel X and SigmaChess 6.1.

My guess is that it is a hardware issue, but a TechTools scan shows all hardware passes inspection. The previous owner replaced the stock graphics card with an ATI Radeon 9600, so I think this might be the culprit - I'll try to find another graphics card to test this theory.

I have done the following to try to try to rectify the situation (none have worked):
•installed new RAM (Kingston - built specifically for G5 1.6)
•swapped RAM in all locations and used RAM two at a time
•removed and reseated video card
•repaired permissions with DiskUtility
•created new directories with DiskWarrior
•started in Safe mode
•updated firmware
•zeroed HD and installed Leopard, then Tiger (both affected equally)
•pulled all periphials
•tried all USB port combinations for keyboard and mouse
•plugged mouse directly into computer USB port
•turned off QuartzExtreme
•reset PRAM, PMU, NVRAM
•created new user account

Any ideas on what to do next? I'm pulling my hair out trying to fix the problem. Thanks!