It's a 2 cell pack, either you're getting the series voltage at the connector (6.6v or so) OR there's something wrong with the battery or wiring between the battery and the receiver plug.
The fastest way to check is plug the balance plug into a cell checker or good charger and see what both cells are reading.
For the record, 5.4v wouldn't make any modern receiver got nuts, thats what you'd see with a 4 cell pack UNLESS its being pulled down below 3v or so with a load on it.