I have a 2003 Echo PB-650 that's been having issues all of a sudden. It starts up with 1 or 2 pulls, and idles great, but it has very erratic operation at full throttle. It kind of sputters and shakes, and sometimes it's randomly cuts out almost, and then goes back up to somewhat normal, but not the full screaming sound that it normally has. The muffler has practically zero carbon, and the spark plug looks normal, with only light tan deposits. Also the fuel filter was just changed. It sometimes doesn't do it, but it seems to do it the most when it's hot outside. I tried taking off the gas cap in case the tank wasn't venting properly, but that didn't affect anything. I am using Shell 93-octane gas with Amsoil at 50:1, and 1 oz. of Seafoam per gallon. All my other stuff runs great. Any ideas?