If you're running the single tail motor, that may be your problem, especially if you are not breaking your motors in. To upgrade to the g90, it's really a good idea to go with belt drive, brushless, direct drive, or dual tail motors with the last 2 being the most common.

Other thoughts might be to check your bearings and the gear mesh and see if they are too tight or need lubed. You might also check your mixer pot. You could also try flying it without the g90 for a bit to see if that has anything to do with it.

I hope this helps out =)