I have done a few uneventful flights with the DF737 autopilot. Recently, i was in the middle of a flight and the altitude hold just decided to quit on me ( I dont' have any failures set). The light for vertical speed and altitude hold comes on with the main autopilot light off, but when i engage the main witch for the autopilot, altitude hold disengages. I am using it with FS2002. I do have the updated version. I never had this problem before.