Help me!! HD problem

What’s the problem:

With 1810x1024 resolution I get 2300 fps and with 1820x1024 I get 125 fps!!! That’s almost 20 times lower fps!! I want to go 1920x1080 'cause thats my 24" Benqs maximum but I get barely 100 frames then! I have sapphire radeon 4870 HD with 1GB memory so I can’t believe it’s about that. Only thing that I painted on screen was rotating image. This happens with GLCanvas which is inside JFrame. Is this known problem?

I am making a game and I would wan fullscreen to work. Theories, anyone ??? ??? ???