You need to check the rcvr batt voltage actually while the engine is running while the trouble is happening. I've seen folks use a data recorder (that's usually hooked up to the esc) on the rcvr battery to figure out what was happening while RC was running - data recorder will tell you what voltage and current was doing while the RC was running.

NiMh voltage with no load is not a good enough way to judge a batt, its the voltage when the batt is under load (connected) that matters.