You can have problems like that if you cross versions.
Panther, QuickTime 6 and FCP4.5 get along. Tiger, QuickTime 7, and FCP5 get along. Crosses many times don't, and can give you the problems you have.
The QuickTime version seems to be the most ticklish.
You can't "upgrade" to Tiger, either. Sometimes that doesn't work. You should clean off the hard drives and install Tiger cold.
Koz