Hi,
I had a c911 completely restored by Omega (actually, mine's a transitional c911 in a c910 case) that came back looking absolutely pristine. Within a couple of months, I had the same issue with the second hour hand-it would "drift" for lack of a better word. I could tap the watch and get this effect once I noticed it.
I contacted a few well-known watchmakers, all of whom said to send it back to Omega Bienne. I was hesitant being here in the US, especially having waited about 11 months on the initial restoration. The watchmakers said that these are troublesome to adjust correctly, and a few didn't even bother trying anymore.
I sent it back to Omega, got it back 3 months later, and all was/is well. It has been perfect now for about 4 years. Well worth the effort to have a nice one, but a bit frustrating in the process.
Good luck
Chris