Try this: If it's freezing up, hit the "PS" button on your controller. This should bring the PS3's XMB screen up, kind of "ghosting" it over the game screen, and bring up the "Quit Game" icon. You're not really going to quit the game at this point - you just want to get to the place where you CAN quit the game. Once it does that, hit the "PS" button again, taking you back to the game.
It's worked for me on several occasions. It seems to provide just enough of a kick in the head to the game to get it going again, and it's a bit quicker to do this than quit the game and go through all the loading stuff again.