Cub Cadet

Hello! Purchased a Cub Cadet Z Force S 60 new in 2012 and have had pretty much no issues with it until recently. Just this season I have replaced the LH Cylinder Head, drive belt, PTO belt, and PTO Clutch. Mowing fine again. Used it three time since clutch replacement mowing 7 acres. This last time got about 5 acres done and the mower suddenly jerked and then slowed way down. With full accelerator depressed it was moving forward very slowly and making a slight squeaking noise. I left off accelerator completely and the mower was still moving forward slowly. Had to engage parking break to stop mower. Turned it off. Restarted, released parking brake and mower started moving forward again without pressing pedal. Quick look at at belt and everything seems to be in order....