Strange Autopilot Behaviour 737
A few days ago, just after reaching cruise level 340 with VNAV/LNAV engaged an interesting "feature" appeared. Another aircraft was just passing us at level 360 and slowly overtaking. Autopilot was supposed to intercept next waypoint by making only a slight turn to the right. However, i felt a mild bump and the autopilot CMD reverted to CWS and started rolling to the left at a slightly higher rate than expected from "regular" LNAV turn. I watched the aircraft and it did not make an attempt to stop rolling, passing already 25 deg bankangle. I corrected manually and reengaged the autopilot. First attempt was HDG mode. Interestingly the aircraft tried to hold HDG while banking 20 deg. Basically flying sideways. In between i disengaged AP again and started to search for a trim problem, also having wake turbulence from overpassing aircraft in mind. LNAV again and aircraft starts banking to the left while trying to fly level. This continued for another 10 minutes and then everything was back to normal. I could not find any trim abnormalities or balance problems. The aircraft above was at this time already well ahead and i still suspect some wake turbulence. But why did the autopilot revert by passing thru this very mild jolt and the aircraft was not able to fly level without banking 20 degrees to the left for 10 minutes?
Any inputs greatly appreciated!
Cheers, hb