Hey guys, new to the forum. I have a mid-late 2009 15" MacBook Pro. As of late, every time it starts up, the desktop will show, and then the screen turns black for +/- 3 seconds. After that, the desktop will show up again, and one of two things will happen: 1) It works perfectly fine, or more often than not, 2) The dock refuses to show up, Finder freezes, and I have no choice but to do a force-restart. Once it boots back up, it always works completely fine. Also, my computer's screen has been going black for +/- 3 seconds even when I'm well in to using it (i.e. it's been on and in use for at least an hour). Any idea what's going on? Thanks in advance, everyone.