Are the club transponders the old, very large ones? Those things can be overpowering to old AM systems so moving the receiver away from it was the right thing to do.

As for other ideas:
Are you running the old brushed silver can motors than many clubs do in F1 classes?
If you are, do you have capacitors on the motor?
If you have the capacitors on, did one or more of them get damaged/overheated during the re-soldering process?

On AM, anything, and I mean anything can cause interference. Even the cheapest brand-name ( example here ) 2.4 system would immediately solve all your issues and make for many more happier trips to the track.