Having monitor problems?

I have some weird problem with my LG Flatron W2243S. When I turned on my computer and monitor, the picture was a little blurry - I sharpened it a little and it was ok, but after some time I notice that the letters aren't what they used to be so i pressed "Auto" to adjust it and it was ok again. Then after a few minutes the letters become blurry again, and I had to press "Auto" again to correct it. My sharpness settings were just as I set them after the letters became blurry again. And while playing games (CoD4) the picutre shakes. How can I fix this?