Aha !

I've grabbed a couple of screen shots of my HUD.

In the situations where the synced layer doesn't run, it shows that it is 1 frame behind the layer it is syncing to. (both movie files are 125 frames, 720x 576 nd were encoded using a Blackmagic DeckLink DV codec)

When it plays fine, it shows that it is on exactly the same frame.

I'll post the images once I can get them across to this windoze box.

Does this explain anything ?

Cheers

Richard