David,
Check all hoses from the pressure side of the turbo, intercooler fittings and up to intake manifold. You definitely have a torn hose somewhere or a loose fitting.

The tear on the hose is opening up when boost comes on and bleeding off all pressure, thereby dropping the power and boost back down. When light cruising the tear is not under pressure, and doesn't open and engine feels fine.

When checking the hoses, don't just do a visual check. Get your screwdriver out and remove each hose and check them all thoroughly. It's likely one of the rubber hoses, the silicone couplings are quite robust and resistant to tears.
If you can't replace the hose immediately (i.e. access to part from dealer), you can wrap it with duct tape, this will keep the hose sealed until you get a replacement. Don't be cheap with the tape, use many passes around and around the hoses and go at least 2-3 inches past the tear on either side.

May the boost be with you,

Andre Corriveau
85 744Ti - current ride
83 244GLTi, 89 744Ti, 94 855Ti
Ontario, Canada