Autopilot Altitude Hold

[raknal]

Currently cruising at 6000, with the autopilot on HDG. Each time I engage the altitude switch the aircraft goes into a steep dive. I have tried to engage several times with the same result.
Have not experienced this problem before. The flight I am on is the restart of a saved flight, aircraft is in excellent condition

Any help appreciated

Raknal

[ClipperLuna]

Some questions:

The aircraft is definitely trimmed for level flight before you engage it? Does it beep at you after engagement? The blue altitude hold light comes on?
Does it happen if you start cold and dark (as opposed to continuing a flight in progress)?
When you start your saved flight, is the aircraft set to Comanche or one of the default ones? Funny things can happen if you start right up with an A2A plane.