mini mac problem

so... here is a screenshot and i think that the problem speaks for itself.
additional information:
- its a relatively old mini-mac (2004!)
- the problem does not always occur, it happens from time to time...
- on startup, there is almost no noise on the screen, but within 1-3 minutes, the screen is being overtaken by the lines and colors.
- the colors and lines keep changing
- i already tried a different monitor and that had no effect, so it must be the mini-mac

anybody has any idea what the problem is and what i can do about it? (virus/ready for format/trashcan?)