Ok, so after I downloaded the new maps a few days ago I seemed to get "Disc Read Error" every 5 minutes and it was unbarable. I used to get it before, but it seemingly got extremely worss. I tried cleaning the disc, blowing in the ps3, get rid of the dust etc. but to no avail. I read elsewhere these instructions and I just played 5 hours straight without any problems.
What I did:
1) Go to your internet browser
2) Go to Tools
3) Scroll down to "Delete Cache" and delete it
4) Next scroll to "Delete Cookies" and delete your cookies
5) Finally, leave the browser and go back to the ps3 main page. Go to "Game" and then go to "Saved Data Utility (PS3). Find your Modern Warfare 3 folder and DELETE it. What will this result in? Your settings for MW3 will go back to default. You'll have to set your horizontal and vertical margins, your sensitivity, and button layouts. In multiplayer your call sign and emblem will go back to default, but ALL of your information will still be there. All your emblems and call signs, all your custom classes, stats etc. It will also delete your campaign(s). Other than that nothing else will change.
For the past month or so I had been getting this disc read error frequently just about every 30-45 minutes, and recently after the updates every 10 minutes. I just played nearly 5 hours without an error. For anyone who tries this, remember you'll lose your campaign progress (who really cares about campaigns anyways = P ), but be sure to post back here and let us know if it worked.
Good luck everyone, hope this helps.