Sounds like the crank was messed up to begin with. This is the one area of the bicycle that takes an incredible amount of torque forces. Buy a new set of pedals and a new crank. Then as you ride your bicycle consider the repair of the crank. "Heli Coils" come to mind. Filling the hole with "Aluma Weld" then re-tapping to 9/16ths. Of course all of this is unproven but just think of what fun...

This is a long video but worth the time just to know for further applications...