Here's a good article on how two transmitters can interact to create interference on other channels. It's possible that you have a transmitter in the area you don't know about, such as a commerical transmitter on 72Mhz. The FCC allows some commerical uses "between" our RC channels. At my field, we have some unuseable channels because there are commerical transmitters that are close to the field and they blanket the RC channel on either side. (if we were father away, it wouldn't be an issue).

http://www.greathobbies.com/document/tech_radio_1.html