B737-800
Human-Machine Interface; Workload; Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft
In turbulence; the PITCH mode of the autopilot dropped out of VNAV path and reverted to CWS Pitch; and the plane immediately started diving at what must have been about 4;000 FPM. I grabbed the yoke; disconnected the autopilot and stopped the descent at a loss of about 300 FT. Later in the flight; the  same thing happened on descent but there was no altitude deviation.Suggestions for avoiding recurrence: Keep the autopilots up.
A B737-800 autopilot twice transitioned from VNAV Path to CWS Pitch without command and the during the first event at FL360 began a dive loosing 300 FT before recovery was affected.