this morning on first yard cranked trimmer (stihl fs 80) idles fine but when i hit the throttle motor bogs and has trouble reaching maximum rpm's. changed spark plug same problem. checked air filter and cleaned element, ran trimmer w/wo air filter with no change. checked filter on pickup, appears to be clean. removed spark arresting screen, no change. gearbox has been greased and gas mix is correct. is there something else i should check? someone told me i need to grease the shaft. could that be the possible problem? if so should i do that or take it back to the dealer because it was bought new in february and its still under warranty? trimmer has approximately 125 hrs.