Yeah, I'd think loctite is the LAST thing you'd want on anything with two such different metallurgies. Rather than grease the threads, you should probably put some (a little dab'll do ya) anti-seize thread compound on them, since the bolt is hard steel and the arm is aluminum/alloy. It's designed to prevent any chemical bonds from forming between the two different metals. And since it's not a "lube", I would guess you'll be less likely to have the threads work loose as you're experiencing now.

What sort of tool are you using on the crank bolts? If it's a plain-old allen wrench, you really aren't able to generate that much torque with it. Check other threads here, or Barnett's or the Park Tool web site, for torque specs, but I think that to generate the torque you really need, you need a wrench with a real handle. In a pinch, you can slide a pipe over the end of your allen wrench to give it a longer lever arm, allowing you to apply more torque.