Try a new Pinion gear or at least new set screw. I use no locktite and never had 4mm set screw pinion come off... Or try to mark where the pinion set screw sits when meshed right and make a small divit (sp?) on the motor shaft so it sits in a little hole and will be less likely to move...
And what body is that?
Maxx - FLM, MMM, CD Med.36x706s
Raja - FLM Ext. MM, 1512, 4s
SC8ight - MMM, 1520, 6s
D8T - MMM 1717 6s