I would start with checking motor case drain volume. Take the case drain line off the motor. Plug the line on the machine side and run a hose from the motor case drain fitting into a bucket. Its important that you do not run the machine without an open line out of the motor case drain - you will blow up the motor for sure if its blocked off.
Not sure if CAT has a case drain volume spec for that unit. I know the newer units didn't. All they could tell me was that it shouldn't be much more than a pencil lead sized trickle with the motor stalled out under full flow and oil at operating temperature. Generally speaking I've found that to be relatively true for most hydraulic motors.
@Nige might be able to find a spec if CAT has one.