Its a common problem with the vacuum wastegate actuator on the R56. Vlad had the same issue with the JCW turbo up top. Take a look at DNA, JZW, RMW and Manic dynos and you'll see it quite a bit. The fluctuations occur too quickly to show up on a datalog but it''s always there. That's one reason we decided to add a boost port tap on the compressor housing. The next round of tuning is set to use an electronic boost controller. In our case well be using a Cortex EBC in conjunction with a Turbosmart wastegate. The nice thing about the Cortex is you can set independent boost levels for all six gears. Plus it stores six profiles. Takes a little longer to set up with a laptop but gives you much better control.