I have experienced problems with the altitude hold function of the autopilot on a recent flight.  This problem also exists in the VNAV function.  When the aircraft reaches the selected altitude, it first overshoots the altitude, then descends uncontrolled.  On rare instances, it could climb itself at 40 degrees of pitch and enter a serious stall.  This might be connected to disagreement between the stab trim display and FS tooltip, with the autopilot and flight model each using a different setting.  As a result, the 767 Captain can only be flown manually, making it very difficult to handle on flights longer than 60 minutes.  It's a shame, really.