I've seen Y cables and servo extensions do that. They look perfectly fine on the outside, but after they were replaced, the problem disappeared. I've also had a lessor F aileron servo lock full up. This was in a flying boat...I knew the servo was iffy, but thought I had resolved the problem.....not! It spun in.

A couple of things you can try is to try a different servo in that position, and see if the problem persists or goes away. Similarly, try that problem servo in a differnt location as see the result. I would really have problems trusting that servo in a jet, though.....