I've seen a few people with this problem before.
Have you installed a new video card recently?
Make sure you have the newest drivers for your video card, and try running in OpenGL or Direct3D for video if you are not already.
That might be because it changes the video modes for your video card, which Warcraft 3 selects the best video mode. Lets put this in a action shall we?
Your video card (Factory Drivers) Modes: Opengl
Your video card (Factory Drivers) Modes: Opengl, DirectX(lets say you install it.)
Your video card (Up to date Drivers) Modes: Opengl(no directX because not supported.
Now Warcraft 3 initially chooses the best mode, but if the best mode at the time before installing your new drivers, which could have increased the performance of your opengl experience, prevented the use of DirectX somehow. But Warcraft 3 doesn't change which video mode it uses. SO you might need to reinstall Warcraft 3