You may need to drop the whole PTO assembly from the mower. I jack the mower up and set jack stands for front and rear to get good clearance under the tractor. Removing the assembly will give you the access to remove this stubborn lock collar. Also inspect the PTO shaft for wear at the bearing mating area. Inspect the bronze bushings the support and control rods go through as well as the rods themselves for wear. Inspect the bushings at the PTO fork. All these areas wear and add up together to cause slippage. We have 6 322/325D's some with 8K hours + and they are good machines. If you intend on keeping the machine and doing most of your own mechanic work get the service manual it will pay for itself.