It may not be an R/C transmitter on the channels you are using, but may be pager transmitters on legal channels BETWEEN your R/C channels.
Find the frequencies of the R/C channels in question. NOT 18, 42, but 72.450 or whatever.
There's a legal pager frequency between each and every R/C Channel.
Let's say you have a problem on channels 17 (72.130) and 18 (72.150)
The frequency between those is 72.140, a legal pager channel.
Then go to this website, under "Frequencies by Frequency, enter that frequency (72.140) and the state you are in.
http://www.phreak.org/html/radio/database.shtml
This will give you a list of all legal pagers in your state on that frequency. Find any in your area that may be close to your field.
It may be due to an out-of-tune receiver in your plane that just happened to hear a legal pager and lock out, causing you the problem.
We had to stop using several R/C channels because of pagers within 5 miles of one of our fields that may have been causing "mysterious radio glitches".