screen gets hazy and goes gray

i have a macbook pro 15" that i bought in march of 2008. it has the matte screen. i have it hooked up to a 22" LG monitor so i can use 2 screens. the past few weeks i've noticed the laptops screen will go gray and pixelated. not sure if i'm describing it right but it's not very sharp and it's as if it's got a gray haze over it and the pixels are enlarged. i do not use the laptop screen for my work screen. i use the 22" monitor for most of my work and the laptop screen is where i open the folders and browse through things. i'm not sure if this is because of any of the latest updates or not. has anyone run into this problem lately? sometimes the screen gets like this for a few seconds sometimes for a few minutes but it will always go back to a normal screen. when it is grayed out you can still make things out but it's very hard to. any suggestions on how to resolve this? also in case it makes a difference i usually leave my computer on and just put it to sleep during the day as i am on and off the computer through out the day.