Oscillations in the pitch trim after AP engagement

[richjb]

Greetings,

I took the B737 for a flight today. After takeoff, I was climbing in LVL CHG and HDG. After engaging the autopilot, the control yoke began oscillating back and forth and stab trim was moving nose up and nose down. I could not tell which control was leading.  You could see the in sim yoke move back and forth while the stab trim wheel rotate nose up and nose down along with the wheel sound. It continued to occur even after I disengaged the autopilot and went back to FD mode.  It was not until I made a manual stab trim input that it stopped. 
 

Best that I can tell, I have no control conflicts in XP, i.e., multiple control axis or button assignments.  Any thoughts on what the problem might be?

Thanks!

Rich Boll