Thanks for your help and advice on this. I have spent the thick end of two weeks playing around with various installs, three computers and multiple settings, particularly under timing.
Here's are my conclusions - noted here in case anyone else find its useful in future.
Going back to Windows XP did initially seem to make a difference for me, but it wasn't conclusive.
With that in mind, I have reverted back to Windows 7 32-bit on my sacrificial capture laptop, as before.
I then experimented with No display, Overlay and Preview. These made no difference to me.
I have followed Lord Smurf's advice elsewhere in this forum and installed Catalyst Center. This allows me to preview playback of a video to see if the sound is in sync there.
What *does* appear to make a big difference for me is the choice of compression. I was using
Huffyuv median before and the sound was out of sync; now I have switched to
Huffyuv left (less compression but faster and less CPU load) and I have now captured four hours from a previously troublesome tape without losing any sync at all.
My timing settings for the ATI USB 600 are as follows:
Drop frames and Insert null frames both DESELECTED.
Resync mode set to DO NOT RESYNC.
Correct video timing for fewer drops/inserts SELECTED.
Everything else DESELECTED.
Don't know if this is something anyone else has encountered but I thought I'd record my findings here in the hope that someone else might find my struggles useful.
Regards to all, and thanks again for your help and advice,
Discy