10.4.10 MAJOR ISSUES

I have a Late 2005 PowerMac G5 2.0 GHz dual core i have been having real problems lately with lots of my software especially games. I recently had a repair where they guys went throguh my computer and foiund some unfamiliar objects that were shorting out my motherboard. I recently got 1 GB of ram extra and updated with an ATI 1900 XT 256 MB graphics card. I've been haveing lots of problems with my games such as Quake 4, Halo, Age of Empires 3: The Warchiefs, and , AA 2.5.0. Now the problems were minor from just graphical glitches to full blown freezing, a stuttering of graphical detail. Now the major problems are now starting to appear when i multitask. Today i had a big freeze, I was playing halo and playing internet radio through itunes and ripping it with Streamripper. All was fine and dandy until i left he computer alone for about 15 minutes. I left it doing it thing, came back and played for about 5 minutes until Halo started to freeze and the audio started to become choppy, it really didn't freeze though in halo it was more like in Slow-mo i could not move but fire my weapon, and i don't know if this is a power supply issues as they guys at apple store mentioned that i may need to get my PSU checked as i've already done. The other thing is i play Quake 4 alone with no other apps running and it will full freeze no little spinning beach ball or anything it just stops picture everything that was the biggining now it loses picture too so it is starting to worry me if anyone could help me that would be great as this would help me alot!
Thanks guys, CompyGenius