My 1996 Evinrude 150 starts fine but spits and sputters a little bit at idle. It slightly boggs down when planing the boat and runs fine at high rpms for a little while.........sometimes 15 minutes, sometimes for 35 to 40 minutes. All of a sudden, it will bogg down and quit running altogether. It will then start but only idle, then will not start at all unless I let it sit for an hour our two. This has been going on for about a year. I have replaced the plugs, water pump, timing position sensor, power pack, water seperator filter, rebuilt the carbs twice. It has been to 3 seperate mechanics that have proclaimed it 'fixed', WRONG! Any ideas.......the fuel is new, the oil is full, the bulb in the gas line is hard.