I'm having trouble with the AP, too  :'( (I have CS727 for FS9 and it worked perfectly.)

I cannot get the AP to maintain altitude properly. The other day it worked and maintained 8,000 feet, but when I climbed to 29,000 it wouldn't work. Specifically, the aircraft begins to nose-up. I tried adjusting the pitch up and down, (and throttle) but in 10 minutes of trying, the aircraft started porposing up and down.

Am I doing something wrong? Can't I just turn on the AP and select 'Alt hold'?