From your description, my 740 behaved somewhat similarly, it turned out to be the ignition key, it was somewhat loose and the play in it was enough to kill the engine, but only for a split second. I'm not sure if you already checked this one or not, but could be worth it. In regards to the launching, the temporary shutoff could have caused the idle control trying to "save" the engine as the engine got the power back. That is if you were going at slow speed, otherwise you would probably not notice it.