I'm going to break with the general sentiment and random interjections seen in this thread and tell you that I'm 100% positive that at least SOME of 'the problem' IS your CPU.
Well, to be more precise, it's the game's usage of your CPU.
What I mean is ... there's nothing 'wrong' with your CPU, but the fact is, regardless of how much CPU power you have (obviously, more is better), if you have a high-end GPU, Borderlands 2 will get horribly CPU-bottlenecked in various areas of the game. These slowdowns to 30-40 fps are most likely due to badly optimized code that runs on the CPU. Possibly the portion of the physX code that runs on the CPU (regardless of whether you have an nV card), but I'm not 100% sure since physX can't be turned completely off IFAIK.
Lowering PhysX to Low will help out a bit with these slowdowns, but not alleviate them entirely.
OC'ing your CPU will definitely help as well.In case you're wondering how I know this, it's because I already tested it out, and posted the answer to the question at handEdited by brettjv - 11/14/12 at 11:11pm