A B747-200 autopilot 'B' entered an uncommanded left turn and did not respond to Heading Mode inputs so autopilot 'A' was selected which corrected the deviation.

Narrative

During cruise at FL340; First Officer was in First Officer's seat. Autopilot 'B' engaged. I went to the bathroom. Aircraft entered moderate turbulence. I notice as I returned to my seat that aircraft was in a 30 degree bank turn to right. First Officer was hand-flying airplane while autopilot 'B' still engaged in command. First Officer told me the aircraft rolled by itself left. At first; First Officer selected Heading Mode to correct deviation. There was no response; aircraft continued to roll left. He recovered from the impending upset and returned the aircraft back to course hand flying it. Altitude remained at FL340 +/- 50 FT. There was no warning: (WAILER or LIGHT). All GPS parameters were normal; Hydraulic System #2 (power to autopilot B) normal. ATC asked us if all was OK; we told them yes. First Officer engaged autopilot 'A' and we proceeded with out incidence. There was no warning: (WAILER or LIGHT). All GPS parameters were normal; Hydraulic System #2 (power to Autopilot B) normal.