I have been seeing a similar problem, OS X 10.6.5 with TM 1.5.10 (no project+), mine does not crash when going back and forth but I am getting the spinning ball when I have files open that are stored on a network volume. It is about anywhere from a 5-15 second wait when switching back into Text Mate from another application.
http://ciaranwal.sh/remate/
ReMate is a TextMate plugin which can be used to disable the automatic project tree refresh when the application regains focus.