I have a G 90 that I purchased from a guy who was having your exact problems and was fed up, got a good deal! Broke it in and never looked back. These engines take a great deal of run-in to be right. (had a G-51 that was the same). I run 15% and
never have had a problem with this engine after the initial gallon or so. Don't expect perfection out of the box with most ST's, good value for the price!