Most of the inbuild FSX autopilots do not work reliably in time accelerations > 2
The pitch of an aircraft is very sensible to autopilot actions and we have put much work in providing a smooth experience in many planes. This is why in the latest version, we introduced autoplit modes that can be selected via the menu of the autoplot window.
Please make sure you use the latest version and select the appropriate plane family from the menu.
If the problem persists please post the plane you have used and the addons that you might have installed, so we can test and report back.
Best regards
rudolf