Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
It seems no one making X-Plane aircraft can make a decent Autopilot. Thats one thing that FSX and P3D had worked out too perfection. You could switch from VNAV-LNav mid flight to ALT-VS-HDG without a blink.
MSFS' default autopilot does not have VNAV capabilities by default. And I remember the stupid altitude capture bug from FSX' autopilot only too well.