To temporarily bypass the switchover valve for troubleshooting, disconnect the line that runs from the intake manifold to the switchover valve, and connect it directly from the intake manifold to the ALDA. If you're back to full power, you know that you need to replace the switchover valve. (edit: see last paragraph)

On my SDL, the switchover valve is located on the driver's side of the firewall... it will be in a similar location on your 300D. You should be able to find it easily by following vacuum lines.

IIRC, there is an electrical connector on the valve that runs to a pressure sensor located in the side of the intake manifold. It is also possible that this sensor is bad. Someone please correct me if this is wrong.

Good luck.