[Serum]

That capacitor doesn't look too healthy! you could try adding some low ESR capacitors to your batteries to see of that solves things.

If the capacitor stopped working the controller gives a pulsating load to the batteries; something NIMH batteries can't stand. Their performance will suffer badly, voltage will drop.. But it also might be something else.

Have you got another receiver you can test? that's one of the things i would try.