Have you tried monitoring memory use with this setting at high or ultra. It's a long shot but possibly you are tipping memory over the top. It's one thing you could take of the list of possible causes.
Definitely not memory. This is the first part of the first level. Vram use is only ~ 1500mb and is not affected by changing this post processing option (also, texture quality and AA cause no affect to the framerate when post proc is set to high or ultra)