Have just spent two hours scratching my head about the same thing.
I am unsure if the solution is permanent but if it helps or provides a clue for someone else, then all well and good.
In my canvas options list I made sure the following were ticked.
Image
Show Overlays
Show Title Safe
RGB
Black
It was when I changed from Black to White the scrubbing problem fixed itself. I returned the selection back to Black and all seems OK – for now.
I don’t know really what I’ve done to sort this out (or why I have had to do so – apart from the fact I have just installed a new video deck) but if it helps someone else, then my job is done.
This is on a G5 using FCP5.
Good luck.