Could be your Exhaust Back Pressure Valve (EBPV, the butterfly valve on the turbo's exhaust outlet) closing. You can try disconnecting the actuator solenoid and see if it helps. Look for a 2-wire weatherpack-type connector down by the base of the turbo on the driver's side. The valve is supposed to be for faster engine warmup, but it's really not that helpful in that regard. Cheers!