I can't offer any technical advice on the subject, but can tell you that you are not the only one. We have 2 FS90R's that are 2 years old now and to be honest have been totally abused. Used only 87 octane fuel and Valvoline 2 stroke oil mix 50-1 and to add to the abuse I work for the city that I live in and we get class D inmates from the city jail on work realease to do all the trimming and never had a problem with either weedeater. Trust me if these guy's can't break it then that really stands for something.
At the first of this spring we bought a 3rd 90R new from the dealer. Been nothing but junk from day 1 and we actually wanted to give this one a better start in life. Bought plenty of the Stihl oil and ran only premium fuel. This trimmer has no power, very slow to accelerate and stalls all the time. Several trips back to the dealer have been no improvement. If we had not had such good results out of the other 2 trimmers then I would have never bought another one, so I don't know what the deal is. When we first took the trimmer back to the dealer his first question was what oil and what fuel we used. He was shocked when we told him, then mentioned how we treat the 2 older units and he said they should never lasted this long. As far as the ethanol, it is a good theroy but I doubt it is the culprit.