For the second time, I have had errors that cause me to crash on departure.

I take off, engage the A/T and SPD hold (speed set to 250) , the plane then sets it to 208, engage LNAV and A/P and the plane says "Autopilot Disengaged" even though I had not engaged it, my first step altitude was 14,000FT, the plane set Vertical Speed to 4100. The plane started violently banking left, I turned off the autopilot and attempted to recover by the plane started stalling and was then uncontrollable so I just pressed ESC and ENTER. I had the same problem like this while using VNAV on departure  once so I no longer use a VNAV departure, so I try and manual departure and it happened again. This NEEDS to be fixed ASAP.