2003 Ford Expedition xlt 4WD. Vents changing positions

The blend door actuator is vacuum-controlled so the first thing you want to do is check for vacuum leaks.

Under the hood, and between the firewall and the battery, there is a one-way check valve with a T-connector that connects three vacuum hoses. I would check the connections here for damage and leaks. I would also follow the hoses to further check for damage. The one going to the left attaches to a vacuum reservoir can on the back of the battery tray. Check this, too.

There's another hose behind the battery tray that is just lying there connected to nothing and can be traced downward under the ac lines. This is part of the hvac vacuum control and should also be inspected.

Any vacuum lines inside the cab dash will need to be checked next. If it comes to it, you can rent or buy a vacuum tester from the auto parts store to check for loss of vacuum to confirm you're either holding or losing vacuum.

Another possibility is a defective vacuum control solenoid. It may be possible that the blend door actuator has a hole in the diaphragm, too, which would require replacement.

Hope that helps. Good luck.