another possibility is that something went bad and filled the pan with crap. Trans runs good for a second til the filter plugs up and then if you kick it off..the crap fall back to the pan and fire it back up you have the trans for a couple of seconds til the filter plugs again.

If you diff lets go.. and fills the pan, this can happen. If so.. swap your diff, clean the pan/filter and you are back in the game.