Okay troy-built Mod# 13AP609G063 42" riding mower 2003. Variable speed drive, changed belts triple checked they were on okay. Mower locked up cannot move except can push in neutral. Variable speed pulleys will not turn. Jacked up on stands and loosened the nuts on the bottom of the variable speed and removed the drive belt. I could rotate the speed pulley and drive pulley. The drive pulley rotates easily and engages the wheels. Replaced the belts and ran on jack-stands. Ran great on forward and reverse. Put on ground and ran 5 feet in reverse then locked up again. Any suggestions.