stihl fs55r?

i have a stihl fs55r ran fine the first year after that nothing but problems. I mix the oil like im supposed to. i have had it back to the dealer about 4-5 times, had new gas line to a new carborator still not running right it stalls out then hard to start. htere is a white film on the screen in the carborator. what is that from and how do i fix this?