91 MM throws itself out of timing

I just replaced the head gasket on my 91 Mighty Max, put everything together and started it up. It idled really high on start up soI shut it off. Then it wouldn't start. Just cranked over with no start. I figured out the crank shaft gears went out of timing. So I fixed the timing and started it up again. Same thing. It went out of timing again. What the heck is going on??

i had this problem before pull the crank gear and check the key way on the gear some one had had the crank gear off and did not tighten it when they put it back on and it wobbled out the gear about 1/4 inch so when you set the timing it would start but if you shut it off the gear would move and it would not start again