Paul
I regularly record long sequences such as concerts, and may have some advice to offer:
I have observed this behaviour when scene detection has been enabled during the capture process from tape. I have also experienced audio drop outs of 1-2 frames between scenes when joining those scenes together on the timeline. When I do not perform scene detection during tape capturing i.e. capture it as a continuous stream, I do not see these drop outs or thumbnail issues.
I also use an AVCHD hard disk camcorder as my safety cam and I have observed similar behaviour with my footage. Unfortunately, my camera automatically creates 2 gb files (does not stream continuously), and it is at the join points I notice 1 or 2 dropped frames or frames carried forward/backwards. The way I work around this is to crop the footage by 1 or 2 frames. It can get quite cumbersome with 20 or 30 clips, and causes syncing headaches every 15 minutes or so when trying to do a multicam edit, but I have been unable to find a more elegant solution.
Hope this helps.
Intel i920, 12GB RAM, ASUS P6T, Vegas Pro 9 (X64), Vista x64 Ultimate, Vegas Production Assistant 1.0, VASST Ultimate S 4.1