my stihl strimmer fs55 will not run properly?

I brought an fs55 stihl strimmer about 18 months ago, it has never been a great starter but this season things have got much worse after alot of pulling I can eventually get it to run but after about twenty minutes it will cut out and not restart, I have various other two stroke tools for which i use the same 2 stroke mix without any probs so i know fuel is not the problem.