Could be a loose/faulty cable somewhere? Possibly between the lav and the tx if it mainly happens when the speaker is moving. Is there any way you can try another lav? - Narrow down the problem.
I've had this problem with 2 different sets of the G2 wireless lavs. It gets very annoying at times having to redo takes because of audio issues. I actually spoke to the Sennheiser guys and the only thing they could come up with was a faulty wire somewhere. The strange thing is that I went through 2 brand new set-ups and both had the same issue.
We had 2 solutions that worked to varying degrees.
1. I had talent take the transmitter completely off unplug everything and put the transmitter in a different place, even slightly different seemed to work sometimes.
2. I would change the sensitivity setting in the menus and then adjust with the on-camera volume level. I have no idea why this helped, but it sometimes did some good.
I dont have the technical know-how to explain why any of this worked, its just my real-world experience with the same problem.
Good luck!