I've seen exactly this before with recon turbos. The actuator seems to react too slow so you get the over/underboost loop. I've managed to adjust the linkage before to reduce the effect but I've never had this problem when using brand new garrett units.

If you log the boost specified, actual and n75 request you'll probably see te N75 react and then a delay before the pressure actually changes.

May not be the same thing but gives you a possible direction.
Where did you get the turbo from?

Matt