I think I have this aswell. Mesa from git updated today. Mesa is compiled with
march=native. Unigine Heaven and Interstellar Marines is where I notice it
mostly, other games seem to work ok.

Everytime I fire the gun in Interstellar Marines fps goes to almost 0 for about
1-2 seconds, fps is pretty good otherwise around 50-90. If there is anything I
can do to help please let me know, I am on IRC as egon2003.