Wierd things on monitor

From a compaq 5712. My kid plays the diablo2 game online a lot.
From time to time, the picture scrambles and he is left with a bunch of distorted colors on the screen. If we ctrl-esc back to desktop, then rejoin the game the problem goes away(until the next time).
Could this be the acceleration rate on the monitor? or just a diablo2 server problem.
This problem only occurs playing this game.