I have the exact same problem, and I have exhausted everything I can think of to try to fix it.
(trashing preferences, repairing disk permissions, resetting PRAM, creating a new project and reimporting media, etc., etc., etc.)
The only thing that I can conclude is that perhaps it is a flaw in STP, which hopefully Apple will fix in the near future.
As a workaround, I just separate the Video tab and drag it onto my second monitor. It's kinda ghetto, but it works.
If anybody actually figures out how to fix it, I would sure like to know as well.
This problem happens on ALL of my Macs, and I have seven G5/Mac Pros w/ STP on them.
Patrick
Tiger 10.4.10
Quad 3.0 GHz Mac Pro
512 MB VRAM
13 GB RAM