I  have upgraded the stock motor and esc to a Castle Creations SV2 Sidewinder and 4600kv motor combo. I also have been running two LiPos to give the system sufficient power. It was all working well until friday afternoon when I first used a ZIPPY Flightmax 2s 35c lipo, it worked well for around 20mins, but then the motor came lose and the pinion gear moved and was no longer interlocked with the spur gear. I then set the gearing right, and tightened the motor screws, it lasted for around 2 mins and then the same happened again. I re-did this 2- 3 times. I then went to use it today, and the pinion gear came off, and then the motor moved again, resulting in a stripped spur gear. I suspect that the motor has moved which has led to the damage to the spur gear. Before I order a new spur gear, are you able to offer any advice as to how I can avoid this in future as I dont want to wreck a new spur gear. Could it be that the extra power is causing the motor to move? 

Many thanks