I had that exact same problem with my old PSU. The Amiga would run for anything between 10sec and 15min between the resets. Disapeared when I cut of the power plug and soldered it to a 230W AT PSU and used that with my Amiga instead (I guess a standard A500 PSU don't support an A1200 with 3 extra drives, Turbo board and Harddrive running of it, at least not for very long). :-D
