Are you using two set screws per crank?

It was only a "common" problem when the flipper shafts are damaged. Then, the cheap ass trick was to swap right and left (new unused surface). And then tighten the crap out of them. I personally have never had to change a crank unless the collar had broken free from the crank bar.

Are these "allen" type set screws?

Are you tightening it to the point where the wrench almost bends and it hurts your hand? Because that's how hard you have to tighten them.