When I try to scrub though anything, fast previews don't work. I have monkeyed with every setting I can find and nothing will solve this. It is quite time consuming to have to use pg up/pg down all of the time.
Mac OSX 10.6.1
Intel Core Duo 1.86 ghz
ATY RadeonX1600 graphics card
if you are talking about live update, where you see the preview window updates as you scrub thru the timeline, there is a toggle at the top of the timeline that switches this option on/off (looks like square overlapped by another square with an x in it).