Oh boy do you have the troublesome CVS Transaxle. It is made by General Transmissions and is a RS800P. Only external parts are available for it; therefore, don't open the plastic casing. Most likely it is a problem with the inversion controls. Note: debris on top of transaxle causes a lot of the problems.

This is their service manual. General Transmissions RS800P

Good Luck figuring what the problem is.

All I know is manufactures are redesigning the new mowers to no longer use this beast.