Strange Autopilot Problem

I'm having a rather strange problem with the autopilot in the default 737. The problem has only begun to appear recently, but when I switch to any view other than the straight-ahead 2D panel view, the autopilot altitude hold fails. All other autopilot functions appear to work, as well as the autothrottle, but if the plane is climbing it will begin to increase pitch up, and if desceding of level increase pitch down, to the point where the aircraft either stalls or overspeeds. Returning to the straight-ahead 2D panel view causes the aircraft to return to the correct pitch. This happens with the default panel, as well as several freeware addons. Anyone else ever see this happen? I can't figure out where to even begin to look for the answer.Thanks,Mel