Weird Autopilot / Go Flight? Problem- Pullin' My Hair Out!

Hello,Something very strange just started today, not sure what I could have changed to make it occur- seems like it came out of nowhere. In short, when I switch on the "altitude hold" mode on the autopilot of any aircraft in FS2004 the plane no longer goes to and levels off at the requested altitude (set on my GoFlight GF-45). Rather. the plane levels out at whatever altitude its at, then goes into a stable shallow dive. As soon as I switch of altitude hold, control resumes as normal.Has anyone every experienced anything like this? Any ideas?Thanks,Joel