Sounds like a bad wastegate actuator and the same issues my car was having. Do you have an aftermarket boost gauge to see what the PSI is reading when it bucks from hitting fuel cut? Pull the vac hose to the wastegate, spray some soap water on the diaphram end where the rod goes into the big cup, and blow into the vac line looking for little bubbles to appear.

You can do this with it on the car BUT it's easy to remove and re-install when finished.