I have a troy bilt tractor purchased new in 2001 model 13AQ609G063 . all the belts are new and the right part # . this tractor has the hydrostatic rear . as I step on the gas peddle the tractor moves foward at a walking pace only and will not move faster . if I step on the peddle harder , it slows down or quits moving all togather. the motor dosn't bog or slow down , just the speed of the foward motion. press on the peddle hard enough and it slips to the floor and the machine stops moving. has anyone else had this problem or know how to correct it ?