The following is from the Support KB #4897:

Q: I have issues with autopilot pitch control.

A: We are aware of the issue, it appears only on systems with no MS Acceleration installed. Because Microsoft missed the AP PITCH HOLD token in FSX SP2, but our A/P requires it.



Fix for those who do not have MS Acceleration installed:
1. Please download the file.
2. Move the file to [Microsoft Flight Simulator X Root Folder]\SimObjects\Airplanes\CS_B727-100\panel folder. Press 'Yes' when asked to replace the file.

From: 727 Captain (FSX) / On the list for next SP (QID 4897)

Check-in to Your Profile > Select product > Customer Support > Search KB or Submit New Ticket, and do a search for KB #4897 and download the fixed A/P gauge.

I think this could fix your A/P problem.

Mark