HELP!!! "A CPU OVERLOAD ERROR" Everytime I track or playback

[EngineeringTheEnemy]

Ok every time I track or play back anything I get this error: "A CPU OVERLOAD ERROR OCCURED. If this happens often, try increasing the "H/W Buffer Size" in the Playback Engine Dialog, or removing some plug-ins.(-6101).
This happens on every session, from the very first track. Usually seconds into the recording and/or playback from previous work. I've had the computer and Protools about 6 months and for the first 3 months it ran great. Then this error started. For the last 3 months I have not used my protools to record, but I need to get this fixed. I've taken the computer in to have someone look at it to see what it might be and the "apple expert" said he couldn't find anything wrong with it. I asked about possible "virus issues" and I was told getting a virus on these apple computers are very rare. Also was told if it was a processor going bad it would just stop working. So i'm hoping someone might have some insight. I'll post my system and info below ... Please help OHHH AND YESSS I adjusted the buffer size just like it said and same result. And there are NO PLUG ins used since it happens right away on first track...

Computer: Apple PowerPC G5 - Dual 2.3 GHz
Mac OS X 10.5.8 version
Memory: 8GB DDR2 SDRAM
Main HD: 250GB
2nd HD (internal): 2TB
2 External HD's (firewire connection): 250gb/each
PRO-TOOLS: 8.0.1 M-Powered
Interface: M-Audio Fast Track