On the left side of the intake manifold, there are two ports. The upper port holds the overboost sensor. This is an electrical pressure switch that controls boost pressure to the Alda device. While this isn't causing your symptom, it's a good idea to remove the sensor and clean it with spray cleaner. There's a tiny hole in the face of the sensor that gets clogged with soot. The lower port is the actual boost sense port. This port is especially prone to soot clogging. You should get in the habit of removing the hose and undoing the nipple for inspection every so often. When it clogs, it reduces pressure at the Alda and prevents proper fueling. Soot can also blow through the lines and the switchover and cause problems. The switchovers for the boost system are on the left wheel well, and it's worthwhile to blow them out with carb spray.

If the Alda system is clear, then the only other thing that can cause this would be the turbo itself. The waste gate is pressure actuated on this car. Although it can stick, it would appear your actuator is working properly. That would only leave the impeller itself. Let's hope it's just the Alda plumbing.

You have a gem. Mine just turned 450K miles, and the drivetrain is still strong. Rust will probably finish it off soon, and I'll miss it when it's gone.