Think about how many cell phones were in that 50 feet.
I was lucky not to be charged (I can't recall if I was under warranty or not), but they did tell me they couldn't find any problems.I sent in an AT 3000 series mic for repair. The unit was dropping completely out every three words (no cell phones on my person). The unit also had a ground loop that other AT 3000 units did not have (it wasn't the sound system). At charged me $60 to tell me they couldn't find any problems. I re-connected, and it worked great.......for one or two gigs, and problem reared its ugly head again. I am ready to jump off the AT bandwagon at this point, because I don't trust their repair services.
Since they didn't find any problems with your hardware, Im betting you are having a mode issue.I had a ton of problems with two of my four new line 6's (one they couldn't find anything wrong with). Don't know what to do. I get so nervous during every ceremony. Wish there was a fool proof system.