I know this sounds crazy but I put a golf ball sized peice of steel wool inside the muffler to raise the tank pressure on a couple I had that was giving the same problem. I done it after gettinbg the advice here on RCU. I was skeptical at first but it worked. You have to use COARSE steel wool.I went through the same thing you're describing, sputtering after 1/2 tank of fuel burned and would not idle which resulted in dead stick landings galore. I later built a baffle out of aluminum for them. You lose just a little power but if the engine won't run with out it what good is it? There is about a 40 page thread in the engine forums on the GMS and Tower engines related to this problem.