I would tend to agree with your thinking.

If it works fine in the shop and experiences glitches on the stage, it sounds like something in that environment is causing interference. I haven't seen any complaints that cell phones cause interference with 2.4Ghz setups though so I would b looking for other source(s).

You are following right turn on procedure? Transmitter is always turned on 1st, then the rcvr. Other wise there can be probs in picking up other signals.