Have you checked that the xtals are seated firmly in the TX & RX? It only takes a little misalignment to make full contact intermittant.

Another thing - and a long shot - this sounds similar to problems we had with Pillow Torque about a couple of years back while using 40mhz (although dual conversion). The only possible cause Geoff could think of at the time was that the venue (Brighton Centre) had low steel beams in the roof and there were a lot of TXs using that 40meg band at the time (robots and model cars). I noticed that Nottingham also had low steel beams in the roof. BTW that weekend at Brighton we had 2 speed controllers go up in smoke - one went up as it stood power on but idle on the bench!