I bought this unit new about 3 years ago and it worked great up until this season. It starts perfectly and runs great for about 10-15 minutes. Then (while still at full throttle) it stalls out completely...no warning, no hesitation first, nothing. I can't feather the throttle or drop to idle to keep it going. It will immediately start back up and run great again for up to 30 seconds, but it will keep stalling unless I let it cool completely. I've replaced the carb, fuel lines, air filter, spark plug. Checked clearance on valves, plug, and ignition module. Cleaned/removed spark arrestor, cleaned magneto contacts on flywheel. I use 89 octane (up to 10% ethanol, but what can you do about that anymore?). Synthetic Stihl brand HP Ultra oil mixed 50:1. The serial # is 292745529. Any ideas???