Dear Rlmarch,
It seems that you are running your engine on the Lean side already when fuel in the main tank is present ;
When fuel finishes from main tank , the engine will run lean on header tank fuel and if you are already at lean side ; the engine will get leaner , heats up significantly and losses rpm (no oil)- and potential damage. Adjust fuel needle valve to run Engine a bit richer (good visible smoke when reving up) and land heli the minute you hear Engine rpm revs up suddenly (without increasing throttle)(which happens when) see fuel in main tank finished. Good luck.