I seem to be having problem with altitude when trying to let the autopilot follow the glideslope when flying the heavies. I hit the Approach button just outside the glideslope flying at about 180 KIAS at around 2500 to 3000 ft above runway alt. or whatever altitude ATC directs me to. The Heading light goes out first and the plane turns perfectly to intercept the runway. But then when the Altitude light goes out the plane starts dropping out of the sky and crashes way short of the runway. The strange thing is this only happes about half the time, I can refly the same flight moments later with the same exact speed and approach and it's fine.
Any thoughts for a beginner flight simmer?