Early 2011 15-inch MBP keeps restarting

The past few days my early 2011 15-inch MBP has been restarting randomly in various ways. Sometimes it happens randomly. It also sometimes won't turn on unless I have the charger plugged in. It seems to be holding a regular charge once I have it turned on and unplug. But just now whenever I unplugged the charger when it's on it shuts down. Then when I plug in back in it restarts again. At this point I can only log into the guest user without it crashing/resetting on me. I haven't done the lastest system update, I'm not sure if this is related or not. I just tried installing it but it crashed again 60% of the way through the update and since then I haven't been able to successfully log in to my main account without it restarting before I can even get to the App Store to update it again. A few months ago I brought it to the Genius Bar and they said it was running fine (aside from my trackpad having a few dead spots) and that my battery was okay (it's at 1,200 cycles now I think). I can't get an appointment again until next week so any at home solutions are welcome!