Laptop monitor sometimes flickers like it's been bumped

This usually happens for a few minutes and then stops. It literally looks like someone bumped the monitor and the entire thing is rocking side to side but the laptop itself is dead still. You can see the picture moving in relation to the sides of the monitor. I have no idea what starts it.

I've tried outputting to an external monitor but the resolution changes and I don't know if that's relevant. I'm still trying to figure out what, if anything, causes it to stop or if it simply stops by itself.

Any ideas?

Windows XP Pro SP3, Dell Latitude D820, should be running all correct drivers. Nvidia Quadro NVS 110M. Resolution usually set at 1680 x 1050 with refresh rate of 60 hertz. Just noticed that I have two Default Monitor and two Plug and Play Monitor entries on my Monitor tab in Display Properties --> Settings --> Advanced.