I have just bought a 3240 hstc with 679 hours on it. When throttle is wide open the RPMS run about 2800. When I have it in "M" or "H" and traveling down the road the slightest incline will decrease in speed to a craw ( 2 - 5 MPH). I cannot pull a 5' bush hog in 12" grass unless I use "L" and creep along. RPMs seem to never change more than a couple hundred, No smoke or oil use. Any idea what may be the problem?