I used to fight this when I first started racing...many many moons ago. Assuming you have all the correct parts, balancer included, and a flexplate that has round holes...not oblonged. You won't like what I have to say but the number one reason they come loose is that they are NOT tight enough. I have helped countless Mopar racers at the track with this very problem! I have used the B&M 8 bolt style flexplates since their inception, before SFI, and currently. I use blue locktight and a 15" snap-on wrench, get the bolt at the bottom, set the wrench so you can push up into the engine while tightening. if done properly you should not have to hold the crank, 60-80 ft lbs and then some!