Video player not working properly

I just installed the drivers for ati radeon hd and my games seem to work just fine, no problem at all. Something happen since then as the movies i try to watch wont work properly. i don't know if is the video player or the movie. I tried watching in .avi and .mkv format and they both have same problem. Vlc and the main video player for ubuntu will behave the same, like lagging or flashing or something. i tried reinstalling VLC and same problem. Any suggestions?