I have had recent problems with my 60hp Evinrude. Starts ok but when I throttle up to WOT, the engine is missing, shuddering and bogging down.. On occasion this seems to clear itself while running and runs normally. I have recently changed the plugs, fuel hose from the tank,checked air valve on the tank and most recently replaced one of the coils that was cracked. After replacing the coil the engine ran perfectly, only to have the same problem pop up again hours after. Before I take it to the high priced help...any ideas??