Try this, unplug your 6v NiMh and plug in a regular 4.8v battery and see if that solves it. If it does then put a regulator on your 6v battery. I use Futaba Tx and Recievers too and the Futaba seems to be a little more fickle about anything much over 6 volts than the others. It's worth a shot and will only take a few seconds. Let us know when you find the problem.

Shawn