I usually have one complete video clip, and two separate clips recorded at different times when capturing my friends music band playing.

When editing and selecting which clip to use (after first audio synching them) the video playing in the main large preview window is always slightly out of synch resulting in my have to try and make educated guesses as to when to switch between clips, based on the audio track solely.

My PC is quite a reasonable specification, so I'm at a loss as to why this might be.

I would add that when I have finished the editing and it switches back to PD16 and I play it back from there prior to producing, the audio to visual synch is fine - it literally is only a problem when editing / selecting the respective clips to use in multicam