I recently bought a 55 SX. When I start it I can rev and drive off ok, however, if I leave it running for a while and then try to drive the revs drop and it refused to move; at that point I can switch off, on again and it will rev and drive ok. If I rev to about 3k and hold it revs drop to about 2.5 and then goes back to 3 again. Pls. advise