Those blowers have a multi-stage timing ignition coil. At start up the timing is retarded and steps up to progressive points of advance as it revs up. The first several years they were out the coils had a habit of popping the internal control diode and sticking it on full time retard and they would act this way. Eventually they worked out a way to prevent the cause of popping, static electricity, and shielding the diode to protect it.
Trouble is, short of some bizzare testing which I wouldn't advise the inexperienced to try due to safety, the only way to verify is to install a new coil and see if that corrects. Suggest taking it to your Stihl dealer, explain what is happening including that you have already checked for clogged exhaust, and that you were advised to have the coil checked for this failure. Stihl warranties the coils for life but not the labor so if the coil is bad you will only pay for the labor and not the rather expensive coil.
Nevada! The last free state!
Retired Briggs MST, Honda Top Tech.
Currently full time experienced novice fisherman.
You should've been here yesterday, the fish were really biting..