Haven't heard any reports of this before. Does it just seem to be a problem during playback, or are mixdowns out of sync as well?
And sorry for the automatic response (now deleted).
Yes, this problem occurs only during the playback.
I noticed that when I work on files that are on an external disc connected via firewire I have no any problem.
When instead I work on files inside an external disc connected via thunderbolt I have... but maybe the problem is that the second disc is a double disc in RAID. So maybe it is a problem due to firmware or so... no idea for now.
With Thunderbolt you have potential contention issues with anything else placing a high demand on your computer's PCI bus - it might not be caused by anything external at all.