Problem-After using trimmer for approx. 20 to 30 minutes it dies and will not restart. Let it set for a while and cool down, then restarts fine but will only run for another 20 to 30 minutes. Had taken to dealer for repair, and their diagnosis was carbon build up in the muffler which they cleaned along with carb. Picked it up to use yesterday and the same problem is occurring. Any help or insight would be greatly appreciated, so that I might help stear them in the right direction when I take it back. Thanks.