Absolutely you do not need PCM, that will just hide the problem. If it doesn't work with FM go no farther.

I one time had a problem like this when I used a hitec crystal in a Futaba receiver, or vice versa, I don;t remember. Check for stuff like this. There must be some common component. Unplug the switch and plug in another battery without a switch, see if that helps.

It will be solved through the process of elimination.

TF