I have 90s 70 yamaha 2stroke, runs great at idle and at half throtle. It will get up and get the boat onto a plain at about 3/4 throtle, but after it runs for about a half mile or so it boggs and the dies. It will start right back up. This spring I had the carbs cleaned out, plugs changed, new empeller installed, compression checked out ok, and changed the oil in the lower unit. what could be causing the problem?