Question about video card problem

Hello all.First, sorry about my poor English.I think I have problem with my video card. (Asus gefroce 2 32MB):- when I fly over the water (long flight) - after 30min the FPS drop from 25 to 14. when I change the MIP quality above one step or below the FPS return to 25 from next 30min.- when I fly over urban area, I see ground texture in high resolution and then low resolution and the quality became high also when I change the mip quality- I tried to remove the render to texture and it cause the simulator crash sometimes with clouds - also I use full screen mode (for high FPS) and If I back several time to the desktop (using alt+enter) and back to the simulator the FPS drop until I change the mip quality- I think something conflict with my video card and cause the video card to stop work.Tanks alottomersystem:P4 2G 512ramAsus geforce 2 32MB