2010 Macbook Pro Suddenly Turns Off

I have a 15" mid-2010 Macbook Pro (2.53ghz i5 w/ OS X 10.8.2) that just started having some strange issues. When plugged into power, it will suddenly turn off while I'm in the middle of using it. This just started occuring today... right after the first time it happened, I tried resetting the SMC to see if that would fix the issue, but then it occurred again a second time about an hour later. Both times I was just doing some casual web browsing, so nothing processor intensive. I don't think temps are the issue, because I'm also doing some casual web browsing now, and my CPU and GPU temps are at 51°C and 48° C. I don't think it's a battery issue either (since I had my laptop plugged in both times), but for what it's worth, I have 880 cycles on my battery.

Before, it used to have this strange issue where the screen would suddenly turn off while I was using it, but usually a keyboard or mouse button press would bring it back up again, so I didn't pay much attention to it. I think this might apply. Maybe that's a possible symptom that led to this?

Anyone have any ideas about what could be wrong? Any help would be appreciated!