I own three of them and never had this issue. Do you have the grounding wire securely attached? Flashed to the latest 1.10 firmware? Everything firmly installed and pressed down in it's sockets? Connections all nice & tight? Tried with a different monitor cable? Confirmed that your monitor works fine?
Typically when you see problems like this start occurring as the device warms up, it's a heat issue. Have you tried with the case removed? Do you have anything else installed around the board that might be blocking airflow? Press down on the top with your finger, does it feel very hot? Blow a fan on it - does that increase the time period before when you start having glitches? Try with a different screenmode - i.e., 800x600 is more taxing to the hardware than 320x200.
Sorry, a lot of options to try, but am sure the problem lies somewhere in there!
