Similar thing happened to me about 4 years ago. 737-300 at FL 360 or thereabouts on a very dark moonless night, FO was PF, quite new I recall. Gradually and smoothly and for no good reason the aircraft rolled left to about 5 or so degrees, stopped, then rolled right to about 10 degrees, stopped, then back to the left again. Only this time the roll to the left didn't stop and at 25 degrees AoB and merrily increasing I disengaged the autopilot and hand flew for a while. The thing flew well enough, all trims normal, so I put my autopilot on and it was fine, tried the FOs autopilot and that was fine too. Reported it on landing and nothing found.