Learjet 35
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion; Distraction; Situational Awareness; Time Pressure; Troubleshooting; Workload
Aircraft Equipment Problem Critical; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Overrode Automation
Aircraft
At FL380; the autopilot began an uncommanded roll to the right while autopilot engaged in NAV mode. I used the correct input to level the wings and disconnected the A/P. Then I reengaged the A/P and noticed the yoke would jerk and I needed to maintain pressure to keep the wings level. The FD was displaying a left bank continuously while both NAV; CDI; and heading bug were centered. The autopilot had the following lights illuminated: heading; NAV; level; REV simultaneously. altitude hold was functioning so I kept the A/P engaged while informing ATC RVSM unable. They did allow to continue the fight at FL380. Upon reaching ZZZ airspace we asked to descend to land in ZZZ.  When trying to disengage the A/P. It would not function with the MSW. I pulled the roll C/B on the captain side and the ailerons did not relieve the right turning tendency. Landed in ZZZ as a precaution in order to swap planes.
Learjet 35 Captain reported a loss of flight control during cruise flight when the aircraft began an un-commanded roll with the flight director and CDIs in disagreement. Control was regained after the autopilot was disconnected.