My 30 hp, 2 stroke Evinrude starts up and runs fine. After I get to my spot and troll, it still runs and starts fine. However, if I've been trolling for any length of time, when I try to move on at full throttle, it misses like crazy and won't go more than about 1/3 throttle. Sometimes, after it cools down, it will run OK at full throttle again. I've had the carb checked, new fuel filter, new plugs, changed tanks, hoses and gas. No change. I've had it to two mechanics for a total of 5 or 6 visits and they can't seem to figure it out. This has been happening every outing since last August. Any suggestions?