To my way of thinking, Heavy's right about the 'Faraday Shield'. It would have to be grounded, not floating to have any positive effect. Otherwise, it is just an antenna for the RFI from the motor brushes. I think the same logic follows for a recvr shield. IE: you would have to tie it to ground (Batt -) to be effective. If RFI is the problem, best bet is .01 to .02 ceramic caps on the motor leads (one from positive to motor case, or the trans case, and one on the negaive to the case).

A simple field strength meter (a diode, a wire loop, an Op-AMP, and a voltmeter) can be used to see how yout xmitr is propagating.

If you are still convinced that it is the recvr, try replacint the antenna lead on the recvr.