Annoying niggle with my DB17.

So I built a DB17, as I alluded too in another thread. Truck is awesome, bulletproof through and through with HD gears, GCM, Innovative RC etc. mods and lst shocks and the install is great looking. Ground down the motor mount so I can run a wider range of gearing to offset the big joes, everything runs real nice and lukewarm on 6s with 17/44 and I've had no breakages or wear on anything. However every run has been cut short by one thing, a whuring sound and a lack of movement as the pinion gear works itself loose and slides off the shaft. Tried blue, red and green threadlock and am out of ideas, no matter what I do it just backs the grubscrew out and stops. Sometimes with sparks if running at night as the pinion bounces off the spur. I'm scared of using superglue on it incase I wedge the pinion irretrievably on a $150 motor. To be honest I was very surprised when red threadlock failed.

Anyway this is the truck: