Sounds like heat to me, too. I'm assuming it happens later in the day rather than early? I'd recommend keeping the laptop off (or atleast in sleep mode) when not in use. Barring that you can buy a laptop cooling pad and that should get rid of any heat issues. I have this one and it works very well for me:
Buy the Antec Notebook Cooler (75004) and other Computer stands, docking & cooling at circuitcity.com
$25 on sale right now, cheaper than Newegg.