On MSI Radeon 9800pro 128 MB
I instaled Fraps for checking my framerate in games. I was just surprised, when the game start (GTA sansandrees, Medal of honor-PA and NFS underground 2) the frame rate shown by the fraps are 60-117 and when i start palying game then framerate suddenly fall at 10-30. Whats the matter. Some time game slow down at high settings but i can play game well in start and after some time framerates falls and game slows down. How can i adjust the constant framerates to the games?(There is no option given to adjust the FPS in games?) How much FPS can my card play at high settings?

You can only improve performance (that is, the number of frames per second that your video card spits out) by adjusting the level of graphics detail (or buy a new card, but you've already done that!). The ingame menus do not require much graphics power, hence the apparently high framerate. Tone down the detail and any AA/AF settings in the control panel.

You cannot "adjust" FPS directly in most cases - the performance you see is governed by the processing power of the video card and the settings you apply in games.

Does my problem due to heating of my 3dCard? Or powersupply?

No, if they were issues then the more likely result would be instability. As I said before, adjust your graphics detail to achieve better performance.