Sometimes when I bring my SURE wireless Lav microphone into a school classroom for interviews, it seems like it drops in and out. It sounds horrible. This does not happen all of the time. I make sure the batteries are changed. Could it be something to do with the frequency being used?
This is not an uncommon issue; I think I speak for everyone when I say, we all have problems with dropouts and interference in all kinds of situations.
You may not be able to find what's causing it...at least in any timely manner. It could be a near-by radio or TV station; I've heard Blackberry's can cause interference. I've had Nextel's and even those strobe lights on police cars cause interference with video cameras I was using. It may not even be a transmitting device; it could be something else. About the only thing you can do is move the receiver around or closer until you have a clean siginal. Sometimes even this won't work.
If your wireless system is "frequency agile" - meaning that you can change the channel on the transmitter and receiver, then I would definitely try that out. You should also try to keep the antennas of the transmitter and receiver within eye shot of one another. For example, instead of hiding the transmitter on your subject, move it so the antenna is visible, and the same with the receiver.
If there's a lot of glass and metal in the room, it can cause the wireless system to drop out. Perhaps you can move things around a bit to avoid the signal bouncing off the glass.
If that fails, see if you can buy an antenna amplification system for your wireless kit. If you don't want to invest more money into this system that is failing you, consider bumping up to a more robust wireless system with a higher transmission power, like a Lectrosonics system that transmits at 100 mW:
I have also done some reading up and noticed that the frequency 696-806 MHZ is subject to interference. There could be a numerous reasons for interference like you said due to the school building setting. :( I might just have the company purchase a new mic and transmitter that does not operate between 698-806 MHZ.
I'll play around some more first.