I was reading an article the other day, which does a fair job of describing the possible interferences between two transmitters and a single-conversion receiver. In the example given, the interference is generated with one (positive shift) transmitter on channel 14 and a (negative shift) transmitter on channel 60. The key (in this case) is the distance between the two transmitter signals in relation to the "Intermediate Frequency" of the receiver so it's not just being on a near frequency that's a problem.
I'm not saying this caused your problem. I have no idea.
But this is a good article for beginners. It explains some of the technical differences between AM and FM, single-conversion and double-conversion, PPM and PCM. And it also explains why there isn't a single "best" radio setup- EVERY system has weaknesses relative to the other systems.
The article is titled:
"Everything You NEVER Wanted To Know About Radios" by Ian Hirschsohn
and can be found at:
http://www.airmedia.se/Receiverfacts.htm
Even if it doesn't solve your mystery I hope it answers some questions for you. It did for me.
Scott