B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Aircraft
In turbulence; the pitch mode of the autopilot dropped out of VNAV path and revered to CWS Pitch; and the plane immediately started diving at what must have been about 4;000 FPM. Captain (PF) grabbed the yoke; and disconnected the autopilot and stopped the descent at a loss of about 300 feet. Later in the flight; the same thing happened on descent but there was no altitude deviation.
B737-800 First Officer reported autopilot dropped out of VNAV and started a sharp descent on two occasions.