Power Lift sticks

My new-to-me 2008 850D came with a power lift. It works but about half the time it sticks and then stops workiing. I have found that if I reach under the partially lifted cargo bed and move the cylinder with a twist (it barely moves) then it will operate again. This makes me think it somehow is barely binding in some way.

Any ideas on how to get this to operate more reliably?

Paul.