I was going to show you a video on youtube, but the idiot simply tightens the crank arm pinch bolts one at a time instead of both at the same time or just turning them each a fraction of a turn until they are both equally snug.

Since you say your crank arm keeps coming loose, then I'd be worried that the mating surfaces have become buggered up if you used it any with the crank arm loose.

You might check that your crank isn't caught by this.....

https://www.cpsc.gov/Recalls/2018/Sp...Injury-Hazards