That’s the problem Patrick. FCP is getting enough throughput off of the main drive to play one layer of video, but placing a transition on the timeline requires additional throughput which your hard drive cannot sustain. Get yourself an inexpensive firewire drive and make certain to capture all your media to that drive from now on. You’ll find that performance will go up dramatically, and you’ll have more real time performance, and no more sticky transitons.
DRW