Hopefully someone will let us know what the actuator should do under normal conditions when idling...

Your problem is probably a vacuum leak or duff turbo control solenoid.

It buried somewhere low down, behind the engine, I think. (I have yet to actually find it!)

The actuator on mine continually moves slowly up then jumps back, when the engine is idling.

I get P0299 underboost codes.

When I find the solenoid, I'll take it off and clean it's air filter.