I know similar problems abound here, but I've tried many solutions and nothing's worked, so I'm giving it one last shot before I just live with this problem or start returning equipment.
We shot several interviews with a 5DMarkII at 23.98 fps with 48khz audio. Recorded audio using a Sony Lav into a Tascam DR-40 at 48khz as well. Encoded footage to Apple ProRes LT, and sequences match the footage setting first.
The sync drift starts after about three minutes. Really frustrating. I've used other Tascam models before and not had this problem. Does anyone have any ideas?
read the manual again and again...
Don't forget the restart FCP part.
Of course there will be a small speed difference between unlocked recorders, but not that big.
If all fails, export the clips to QT audio only, wave or aiff, see if that works (that should remove the timestamp that fools FCP)
Since you probably don't have any use for the BWF timestamp, you'll be fine.