Had a similar loss of power which was found to be caused by the plugs of one cylinder fouling with carbon during a 5 hour flight. The problem was the Cee clip holding the port (left) needle had come off and the needle retreated to the top causing a very rich mixture, coating the plugs with so much carbon it shorted the plugs. The other cylinder (further from the carby) on this side also had a carbon build-up on the plugs but not as bad. Cleaning the plug after re-seating the needle and the engine ran normally.
