More on this. I posted the video clip to rennlist
here.
The engine is effectively rev-limited (at 3500) when this happens. You can see the boost gauge fluctuating. It's acting like the overboost protection is intervening, but the overboost sensor has been disconnected, soooo....
Car runs 1.0 bar happily at sea level. Again this problem only occurs at altitude. It also blows black smoke when this happens so it's (also?) running rich and/or misfiring, too, FWIW.