I have a zero turn cub cadet mower (TANK) 60" cut it doesn't cut smooth like it did when it was new about 6 yrs old now I put on new blades, new rear tires and measured everything that I could and everything seems just fine but some times looks like it will cut a swath on one side that is atleast an inch lower than the rest. doesn't do it all the time seems like but just enough to make my lawn look bad. What could my problem be?