21' iMac (Late 2009) Keeps Freezing And Restarting

I recently just upgraded the RAM. I put in 2 4GB chips and left the original chips in so now it's running with 12GB of RAM. Soon after that, it started freezing and restarting. I don't even have to be doing anything that straining to it. Even just running iTunes (playing music) and Safari (Facebook) can cause it to freeze and restart. It can happen 3 times in 7 minutes or once every 5 days. Sometimes whenever it reboots a screen will come up that says "Your Mac restarted because of a problem." I also just recently upgraded to Mountain Lion. So does anyone think it's Mountian Lion or the RAM? I made sure the RAM chips are inserted correctly. Should I take back the RAM and get new chips (went to Best Buy and got the exact kind needed)?