7550 Black Screen

I have a Precision 7550 laptop. At the beginning of December I started having two problems. The first is my computer starting freezing. The second is I would power it on but only get a black screen. The keyboard would light up and the fan would speed up and slow down as if it was trying to start up, but still a black screen. If I waited long enough with the computer off it eventually would boot up normally but I would have to wait several hours. I addressed the freezing screen issue by replacing the RAM which I determined through testing was corrupt. I thought that would fix the boot issue too, and it did for a while, but now that is back. I know my screen works because when I hold down D and hit power the screen cycles through several different colors, but it still won’t boot up. Any help is appreciated. Thanks. 

Tom