Thanks Jim, but that's going to be hard to organise for me, as I imagine the problem could only be reproduced in a project that contains a lot of P2 footage. In any case, reproducing the problem will be difficult as the artifacts occur infrequently.
I just hoped that someone had already come across the issue in their own work....perhaps while using a different video format.
One thing that occurred to me is that it might be due to streaming the files via USB 3, from a decent though unreliable drive (Western Digital MyBook).