Assuming the oil is OK and the cable is not the culprit:
If the issue developed over time in a gradual manner, I would suspect the liners.
If the change happened from day to day I would expect a mechanical damage to one of the components.
Both liners issues and mechanical failure require taking the box out ;-(.
I have read of many such cases with MS and MSB - as they grow old they sometimes refuse to engage in on of the positions when the oil gets warmer.
The box can be removed and opened for inspection
. A good garage will re-do the liners but if any mechanical failure is found, some original VP parts
are no longer available - then repair depends on ability to source or manufacture the replacement part. There is an online catalog that quotes availability and the prices too.
If there is a VP shop in your area then and experienced VP mechanic
will be able to tell you the exact cause without taking the thing out. AT times, they will tell you what the issue is just by listening to your story. 99% of the time such a diagnosis is well worth the hourly they charge.
I am not a mechanic
just a user so listen to all other inputs before you commit to removing the box.