[MotorHead]

PID is a great way to go for maintaining position. The trick is just getting the tuning parameters correct. I don't think it is necessary to control power. Controlling position is what you need. What you describe with the arm moving is a common problem. Start tuning the PID controller by just setting proportional control. For many systems, this is enough to get the system to be stable and respond to changes in inputs. What you will notice though, the weight of the arm will cause a persistent error. To make this go away, use a little bit of integral control. This term has the effect of adding up error over time and the controller will respond to eliminate the steady state error.

With some patience, I think you will like the results.