String Trimmers

Brand new 224L trimmer, oil (came with), relative new gas. Start-up procedure went smoothly/choke full, engine idled smooth after warmup/choke off.  A few hiccups on first full throttle, attributed that to break-in.  Finally would hold at full throttle and off to trimming I went.  After the trimmer was warm/hot (10-15min of full throttle trimming, farm field grass - not wimpy grass), the RPM's rolled off at full throttle and the engine died.  Restarted easily, maintained low idle.  Would full throttle for 1-2 sec, then roll off and die.  Repeat.  Engine works fine when cold.  Local repair tech for Husqvarna could not find any Throttle adjustment other than the user-available idle screw.  Worked great for him because it was a cold start/run. Because of the warm-up time issue this will be difficult for the shop to reproduce.  I need to get warranty repair done on this.  Any ideas on my next steps?