A Cessna 750 suffered altitude and course deviations when the autopilot malfunctioned at FL430.

Narrative

During cruise flight @ FL 430; with autopilot engaged and NAV/ALT selected; aircraft banked right and pitched down with no autopilot aural warning or other warnings. A 15-degree angle of bank resulted in an initial course deviation of 20 degrees; and a 10-degree pitch down resulted in a decent rate of 2500 fpm and an initial altitude deviation of 500 feet. Pilot flying immediately disconnected autopilot and smoothly initiated a return to assigned course and altitude. ATC Denver center was immediately notified of the autopilot malfunction and resulting deviation and stated that there were no traffic considerations in our area. With ATC clearance; the recovery was initiated smoothly so as not to alarm the passengers; resulting in a maximum altitude deviation of 1500 ft below assigned altitude and a maximum course deviation of 30-degrees of assigned course heading. Crew returned aircraft to normal flight on course and altitude and completed the remainder of the flight without event. Upon landing and securing aircraft; the company was notified of the in-flight event by PIC.