Interdependency between applications at Pbuffer problem

I observed, that an Pbuffer problem in my application (bad colors at some Pbuffer pixels near the corners when multisampling is on) depends on other applications: When another OpenGL application (GLUT or Win32, without Pbuffer) is windowed (not iconized), then the Pbuffer in my (active) application is ok.
Using Windows2000, GeForce FX5900XT.

I guess an initialization problem, but after hours and hours of debugging and comparing with other applications I’m stumped.

Any suggestions & ideas?