Hello! I have a mid-2010 MacBook running OS X 10.8 Mountain Lion, and it heats up very quickly, even when it's fully-charged and not plugged in, when I'm just doing some light reading on the Internet. I find that the fan speeds up and slows down at random intervals, and sometimes, the computer just freezes, overheats and/or crashes. Most of the time, it just becomes quite laggy. This is usually alleviated by closing the lid of my MacBook, waiting for it to go to sleep, giving it a few minutes to cool down, and then opening the lid again and resuming my work as per normal. Otherwise, in more serious cases, I usually shut the computer down (occasionally, I have to force the computer to shut down by holding the power button) and give it a few minutes to cool down, and then I restart it and begin a fresh session (I don't re-open the windows I had closed ih the previous session, for instance). I have taken my MacBook to the AppleCare Helpdesk at school (I use my computer for school) a few times before, and they have replaced and reset certain components (I wasn't informed which ones). These days, my MacBook hardly overheats. I would suggest you take actions similar to the ones I have taken over the past year-and-a-half since the issues started. Your MacBook might need its PRAM to be checked and reset, or its fan replaced. Alternatively, you could download smcFanControl (available free-of-charge here) to control your fan speed and cool your MacBook down when it overheats or is close to overheating. You could also open the Activity Monitor in the Utilities folder and check if any application or plugin is overtaxing your computer's CPU/GPU. Good luck solving your MacBook's overheating issue!