This may get more complicated than you might want to deal with. There are a multitude of things that can cause your problem. Looking back at my own situation, I thought I could diagnose and repair my own MB. Now, four years and thousands of $$ later, I'm no closer than I was. IMHO you'd be better off taking it to a reliable tech. At least find someone with the proper equipment that can properly identify the probable cause and then decide whether you're ready to repair it yourself.