I don't know anything about your specific Robertson drive, but in general, these hydraulic drives have a reversible
electric motor driving a hydraulic pump that is engaged by an
electric solenoid controlled valve.
If you are hearing noises from the
motor, most likely it needs servicing. Sixteen years of moderate to heavy usage is about the expected time for them. Like any
electric motor of this type, it has
brushes and bearings with defined lifetimes. These motors are very easily serviced and
brushes and bearings are relatively inexpensive. Depending on your skills, this may be something you can do. If not, any good
electric motor shop or, probably, autoelectric shop can
service it.
Brushes at end of life could explain your loss of
power and wandering. Grinding noises could be bearings or a brush end bottoming on the commutator. Clicking noises could be the solenoid engaging as normal, then disengaging because the motor is drawing too many amps or not responding at all due to needing
service (that would be expected behavior for it).
It is unlikely the actual hydraulics are the problem.
Good luck,
Mark