I've got a 2011 335i xdrive with the Dinan stage 2 tune, catless downpipe and corsa exhaust. Under very light load, light throttle (boost just coming on) the boost oscillates. The car gets into a rhythmic surge. Less throttle and it goes away, more throttle and it goes away. Typically this is in the 1400rpm range in a fixed gear and light throttle. Car runs perfect everywhere else and runs strong.

Has anyone else experienced this? Anyone know what the cause could be?