Starting a new game shouldn't affect your multiplayer stats. With that said, your multiplayer stats are not tied to your PSN either. I was level 43 and my old phat PS3 died so I got a slim. Had to start from scratch on multiplayer because I thought it was tied to my PSN and didn't bother to copy my saves. I'm not sure if it is tied to your settings file or something but make sure you back up everything every now and then so you don't end up like me.