I created my own class because I want to wrap all drawing methods. When there's enough time, I want to implement another class with opengl which can be used optional.
@Lifepower: I thought that the Antialias property was removed in the new version.I'll try your code. Maybe that's the solution.

@Huehnerschaender: Both pcs have win2k. One has a geforce4MX, the other one a geforce2TS.

Here you can download a demo of my game. In the file "Cobal.ini" you can set properties like Fullscreen, BackbufferCount and VSync.(~36MB)