As Nath said, spark arrestor is a possible culprit, as is the coil - though it will often just die after getting hot when its a coil
As I am pretty sure the fs90 is a 4mix stihl, I would be tearing in and checking the valve clearances.
For those readers who are unaware of the stihl 4mix, it is a four stroke engine that still requires 2 stroke mix
Shindaiwa do a similar one
