My 09 did the same thing on the dyno. I was at McNews getting a N2o set up tuned at the time. Ray was able to tune the issue out. He said he had the same prob with a few stangs. If you have problems fixing the issue I would suggest emailing or calling Ray at McNews and talking to him about it.
FSWerks stg2 w/ water/meth; Cossie intake.