Hitachi Ex60 rev problem

Have a hitachi ex60-5 which is reving out fine but studders and drops revs after a few minutes. It will drop revs completely and will not rev back up until you physically turn down the rev dial and turn it back up and repeats. Never cuts out or makes an attempt to will always idle fine. Have changed fuel filter, cleaned banjo bolt screen, checked pipe for collapsing and blew back into tank .. all seems fine. Any help appreciated I was maybe thinking would the contacts in the the rev switch be giving trouble perhaps