The beginning of some regions drop out

[Minty]

Hi I'm running Pro Tools 8 LE 003 on a mac pro quad and I have two problems with audio glitches.

1/ Occasionally, I will have a short session, (for instance a 30" tvc) where the start of a region within the session doesn't playback when bouncing to disk. In playback it's fine but glitches in bounce to disk, especially noticeable on the VO track. I end up having to re-record back into Pro Tools and export the region as my master file.

2/ On larger sessions, (say an 30 minute tv show) I occasionally get the start of regions dropping out in playback. I'm baffled as to why this should happen and can't recreate the problem because it's not consistent.

No other apps are open and the sessions are reasonably lightweight in terms of tracks and plug-ins, certainly nowhere near maxing out and there is no activity on the Task Manager window when these drop outs occur.
Of the 8 processors Pro Tools is using 7
Playback engine is set to 85%
DAE Buffer level 4
HW Buffer 1024

Anyone come across this problem before?