I got the problem, that opengl displays are disappearing. On the system runs a CAD-System (mem-usage 360 Megs) and my application (mem-usage 100 Megs). After a few switches and operations between the two apps, both openGL displays are disappearing. That means, they are blank and filled with the background-color. Can the memory-usage be the problem?
Resize of the openGL window won't help.

256 Megs Memory

Complete memory usage:
About 600 Megs (Cad, Myprog, 2 Visual C++ etc.).

Any ideas?