Captain reported the autopilot commanded an abrupt pitch up during departure. Captain regained manual control of the aircraft; re-engaged the autopilot and continued the flight; citing software issues as contributing to the event.

Narrative

Selected L autopilot on at 1200 [ft.] AGL; 230 kts.; slats retracting; VNAV engaged at about 5 degrees of pitch. When auto pilot became active; it directed an abrupt positive pitching moment. Passing 18 degrees of pitch with no sign of stabilizing; I deselected the autopilot and recovered the aircraft. No alerts of any anomalies displayed and stick shaker did not activate. When I took manual control of the aircraft after the event; the aircraft was significantly out of trim nose up. I considered raising the guarded switches and cutting out the trim; but I ascertained quickly that I was able to trim out the stick forces without being overridden by the trim system. After I stabilized the aircraft in normal flight; I engaged the L autopilot again and noted no other anomalies for the remainder of the flight. I wrote the event up in the [software program] and read a few hours later that the L autopilot was deferred. This event could have easily been a stall and recovery event if I had put the autopilot on and become occupied with another cockpit task. As it was; this event was an unusual attitude recovery. My assessment was that it was not the autopilot; but the inferior; substandard software logic of the MCP and FMS. Anomalies of unpredictable autopilot and auto throttle behavior are common in the 767. The entire software suite needs to be reworked by the manufacturer.