I have a 2011 Husqvarna GTH26V52LS mower with a Kawasaki FR730VAS04 engine with a little over 90 hours on it. It has run great but recently at idle and intermediate speeds the engine began to surge but it doesn't die. At full throttle for mowing it seems to run fine. I have only used 100% gas in the mower and over the winter months put Sta-bil in the gas tank. I've changed the fuel filter and used spray carburetor cleaner but to no avail. Any idea what could cause this problem and how to correct it?