First
Turn off "Improved L4D2 Decals" and "Improved Blood Textures" add-ons.
Also turn off old version of Andy's realism mod[l4d2.gamebanana.com].
Campaign has a conflict with this add-on. We are going to eliminate this conflict in the next updates till than, turn off this add-ons.
Second
If “Paged Pool Memory Available” in video options set on “High”, set this option on "Medium" first and if doesn't help - set it on “Low”. Also try to turn off “Multicore Rendering” in options