Like so many others, I'm experiencing the same problem.  I have the fat original PS3 and have played COD MW3 without issue for months but all of a sudden it just started freezing during multiplayer online mode either immediately after a game (when viewing the scores) or right before a game would launch.  I thought it might be over-heating, but it was happening within just a few games. I took it in anyways to a reputable service center and they upgraded my fan (very loud now) and then did a whole bunch of maintenance checks and refreshes on the system and said it was good to go.  Nope... still happening and now after the first game.  I have tried lots of other solutions like wiping out the game data and having it reloaded, cleaning the lens with an expensive lens cleaner, laying the unit on its back or side (mixing it up), nothing mattered.  I then tried playing COD1 online again and experienced the exact same thing which told me it wasn't the MW3 game, but something else with the software or system when goes online.

I still have no fix and now I can't play MW3 online anymore... too many reboots and restarts are starting to affect the blu-ray players ability to read discs altogether I think.   Thought about buying another PS3, but only if I knew for sure it wouldn't happen again but I'm not convinced its the player... i think there's a code error somewhere.   I won't be buying any future COD MW games though if this problem continues. 

THAT'S WHAT KEEPS ******* HAPPENING TO ME, IT'S VERY ****** ANNOYING I CAN'T STAND IT..i have the exact same problem as you do. WTF DO I DO?