That can be one of several problems, including a bad picture tube, but it's most likely a bad solder connection. Most of the AST monitors were made by Acer, which was known for that problem with that vintage of monitor. It will probably turns out that the board connector pins are poorly soldered to the printed circuit board. As the unit warms up, stuff expands and the connection "heals" temporarily. Unless you fix it yourself, it may not be economical to repair.