Stutter in windowed mode

Hi all!I realize much has been said about stutters and I understand well what they are and how to avoid them. But in my own case I'm simply out of ideas and several hours of web searching brought no result.As the topic name suggest, I've got stutters but ONLY when in windowed mode. They come up in very stable period. About every 4 secs the simulation freezes for quarter of a second. Any aircraft, any place on Earth. Full screen mode runs very smooth. If any would have any idea why this could happen, please help me out.I've got laptop, Core2Duo T7200, 2GB RAM. On the secondary video I've got TH2G providing 3 external views covering about 210