I have a 25 hp. Mercury 4 stroke--shop has rebuilt upper engine -head gasket-fluxed head rebuilt-carb-have water/fuel seperator-did leak down and compression test( all fine) problem-after couple hours of slow trolling -go to full throttle engine losses power -blows out white smoke from exhaust-( smells of oil)-takes a while and can bring it back to full power--does not do this any other time-shop and Mercury say they don't have a clue--any help??-has low hours boat sat for many years in a garage-