I took the plunge and bought my first Stihl trimmer. Very different beast from the Redmax trimmers I've been using. I have run 5 tanks of quality fuel with Stilhl HP oil and I have a concern regarding the smoothness of engine. 3/4 throttle and the unit runs nice and smooth, but full throttle and the engine housing starts shaking violently and sounds like it's missing. Full throttle in heavy growth it runs fine, but when it's only light trimming duty it starts acting up. I also switched to some .105 line and that seemed like it added enough load to smooth it out some at full throttle. I took it back to the dealer to have them look at it and 2 minutes later they brought it back and said all is fine. I've also noticed that it will stumble for a split second when testing from various throttle positions to WOT. This is the first 4mix tool I've laid my hands on, so really not sure if it's truly and issue or not.