Disc Read Error - Please Help

I've had MW3 since launch. The game essentially has stayed in my 360 tray with very few exceptions (occasionally I would pop in BF3 or Halo Reach, but not all that often).

In the last two months I've been getting a Disc Read Error while playing MW. It only happens when I join a multiplayer game. I can load the game's intro, the menus, mess around in the barracks, all that - just as soon as I get about 30 seconds into a MP game, I'm interrupted by the read error.

Now this doesn't happen every single time I try to play online, but it's starting to become more frequent. The disc doesn't appear to have any severe scratches - like I said, it's mostly been sitting in the tray since November.

Does anyone have any ideas or tips? This is really starting to upset me, MW is the only game I consistenly want to play.. I've put a lot of game time on this mother, could my system have eaten the disc up? Is there anything I can do? I don't have a big enough harddrive to install the game, obviously don't have a receipt.. Should I try to take it to Walmart and explain that it's broken?

Any input would be very appreciated, thanks -