In case anyone cares I have an update on this. I was able to get a
crystal controlled RF module for the transmitter and that took care of
the problem. It's definitely the synth RF module that's experiencing
the interference.

I'm still interested in findind a way to deal with this aside from
just not using the synth module at that site. Now that I've more or
less isolated the problem, is there anything I can try? I did make an
attempt at sheilding just the RF module with no luck, but are there
other options?

Thanks.