Yes, intermod is another possiblity. Each of those transmitters may run many kilowatts, and there would certainly be opportunity for creating intermod. The intermod could arise at or near the transmitting site, say a corroded bolt on one of the towers or maybe even in the circuitry of your receiver?

The advantage of a spectrum analyzer is that it should be relatively clean of images. You can also set the bandwidth to be very narrow. This will allow you to tell whether or not another signal is really on your frequency. I don't think the R2 will let you set the bandwidth below about 10 kHz ( ?), so a signal located properly on a channel inbetween the RC channels may appear to be on an RC channel. The other thing that is helpful is that you can sweep the frequency and see what else is around that could be a problem.

Hams deal with this sort of thing alot. Some hams even enjoy tracking down RFI problems and consider it part of the hobby. They typically use directional antennas to locate the RFI source. It does take time and skill, and some specialized equipment.

Hopefully the problem will go away when you replace your single conversion receivers with dual conversions. Using a single conversion receiver in such a busy RF environment is possibly expecting too much.

Good luck!