The second belt Star is talking about is Hydro belt #21549025. You may have replaced the engine to hydro belt, but you never said you replaced the other belt. If this unit came to me this is what I would do. You may need an assistant to do this. 1. Remove the battery and battery case. You should now be able to see the top of the hydro. 2. Remove the seat safety switch from the seat and clamp it with a small C-clamp to close the switch. 3. Jack up the rear end and support it on two jack stands. 4. Use jumper cables to connect the battery to the battery cables. 5. Set the parking brake. 6. Start the engine and release the parking brake. You should now be able to observe the hydro operation through the opening where the battery case was. 7. Have an assistant operate the foot pedal while you observe the hydro operation. Go back and forth, between forward and reverse, both freewheeling and holding the tires. You will be looking to see if anything changes, particularly during reverse operation.
I’m thinking you will find that the hydro belt is slipping due to wear. You and others may disagree with this procedure, which is fine, just thought I’d give you my way of diagnosing tranny problems.