Little Big Planet 2 Loading Problem?

Okay, my little sister is a fan of the lbp series, so she was playing it earlier today (as always) and she wanted to play LBP1 for a little bit. After she played, she let it rest. (Our PS3 has an overheating problem, you can only use it for 2 hours and let it rest for 30 mins or else it will shut down with a "beep beep beep!") Later she came back to play LBP2, she put in the disk correctly, and it started up, but the loading at the beginning took about 8 minutes. It never takes that long, it takes only about 15 seconds. After the loading finished, she was sent to the menu (or the pod.) And when she selected a level, it started to load for a long time like the beginning loading screen. It wouldn't load correctly. She told me so I thought that it was the disc that caused the problem. I was a little white mark on the edge of the disc. I wiped it off, and the disc was fine, along with the few tiny scratches. So we tried it again, still didn't work! I wanted to see if it was a problem with the PS3, so I played a few other games, they worked perfectly fine. (Including LBP1) I went back to LBP2 and was stuck on the stupid 8 min loading screen so I quit the game from the menu that pops up when you hold down your PS on, and the PS3 shut down with the Beep beep beep! And then it rebooted back to the user selection. It goes beep x3 whenever I try to quit the game. Have I done anything wrong, or is it just a simple overheating problem? (My sister also says that it sometimes freezes while into 1:45 into the game.) Thanks in Advance! :)