Inconsistent Tempo/Playback slows down,speeds up

[perfectdrug6]

Hi everyone,

I'm running Pro Tools 8.0.1cs2 on a powermac G5 dual core 2.3ghz, OS X 10.5.8. The problem is the playback cannot hold a consistent tempo, so if I'm trying to create a drum loop, whether it be audio or midi, fully quantized or not, I can't retain the groove, rhythm. I can see the fluctuations clearly in the main counter or big counter, bars go slow then fast, and it seems completely random, even on a completely blank brand new session. I'm not getting any error messages. I'm not sure if this like a sluggish playback that catches up at various points or what. Any advice greatly appreciated, if you need any more info, please let me know.

Also I'm on a digi 002, lacie extreme triple interface external HD for recording, everything connected 6 pin firewire. Internal clock on PT. No third party plug ins, just what came with Pt.
I've tried:

- reinstalling OS and pro tools
- repairing disk permissions, repair disk
- changing firewire cables
- different h/w buffer settings, sample rates, all kinds of preferences
- trashing prefs