This had been puzzling me for months.
Whenever I tried to send a large file (over 5MB) from Treo 650 to PC via bluetooth, it reset the phone. #*377 always reported the program I used to send the file, be it Zlauncher, FileZ etc.
Seeking help from PalmOne, they advised me to upgrade the firmware. When I aksed them if the upgrade fixes the problem, they said it's not a know problem.

I finally found out that Butler was the culprit. When the following functions are enabled, I can't send large files over bluetooth (can receive though).
-Attention: Incoming call
-Attnetion: Incoming SMS
-Hide SMS
The phone resets at "preparing" stage.

After disabling Butler, the reset problem is gone.