Jerky PLayback

ASUS/P4B533 Mobo, P4/2.4, 512M, 450 gig RAID/0 and 80 Gig Boot drive,
Audigy/2, Win XP Corp SP1 ONLY ATI 8500/128 Meg, MMC 7.7 and original
ATI CD Drivers.

I have the room to record un-compressed avi and it's been beautiful
for months, I recorded an MPEG-2 via MMC and noticed jerky playback,
as I look around my avi's also played back jerky - not every x seconds
- random.

Full re-install with the CD drivers that came with the card as I know
they were stable.

The same scenario.

BUT... if I copy the file to any of 4 other machines, including my
dandy little Celeron/600 Dell notebook, the playback is flawless.

I've gone through everything including resetting the BIOS and the same
result. I've another identical system but with a Radeon 65 AIW card
and it won't play them back smoothly either.

I'm killing myself over this, the odd thing is that if I use the
time-shift and pause it for 10-15 minutes then come back and continue
the playback is smooth as silk.

I'm lost (as usual) - any ideas out there - short of an XP re-install?