Aircraft will not level out using autopilot

This problem has only just started, so I'm wondering if there is a setting I have inadvetently altered somewhere.
If I use autopilot on any of the FS 2004 jets to climb to a set altitude, the nose remains too high when it reaches altitude, the speed drops and eventually the plane stalls. At no time during the climb can I get the speed above 320.
I have reinstalled the simulator several times!
Any help would be appreciated.
If I use autopilot on any of the FS 2004 jets to climb to a set altitude, the nose remains too high when it reaches altitude, the speed drops and eventually the plane stalls. At no time during the climb can I get the speed above 320.
I have reinstalled the simulator several times!
Any help would be appreciated.