Dude, I don't want to put the wind up you here, but the problem you are having is the best way to completely destroy the bottom end of an engine I know of.
Firstly, loctite +11ty (although it will be a bolt, not a nut) Keep in mind that the thread you run the bolt into is actually the nose of the crankshaft so be vewy vewy careful of the condition of the thread on the bolt - it is a grade 8.8 high tension jobbie that can mess up the internal thread on the crank nose if you aren't careful.
Secondly, have a really close look at the thick washer behind the head of the bolt as you are running it in and make sure it is locating properly, I have seen this happen before where the washer was sitting up on a ledge and when the bolt felt tight, it was actually deforming the washer.
Lastly, I always use a rattle gun when I am tightening a harmonic balancer, apart from the fact that you don't have to risk chipping cam gears etc when you are trying to stop the engine from turning over, impacting it tight will hep the loctite cure properly.
Oh, and obviously make sure the key and keyway are in good condition and the rubber ring is not showing any sign of wear as this will lead to increased harshness and vibration.