I also did purchase a Pack 4 on steam to play with my friends and apparently the problem is only on my game. Always after 15 to 20 minutes of starting the game features a frame lag or miss absurdity.
I use a system Phenom x3 720 2.8ghz, 8G MEMORY and Radeon HD 5830, already reduced the game resolution to 800x600 and the problem continues both in single player and cooperative games.
I tried to do the checking game cache on steam and I always get the message "1 Files failed to validate and will be reacquired." and never seems to be that the file is corrupted repaired.