Thanks for the reply Arthur.

I've checked the purge solenoid valve. It is clean, seals well and opens when powered. I can blow freely through the vacuum tubes.
The valve 'beats ' as you described.

The idling speed drops rapidly when the cam adjuster is activated.

Code from pin 14 is 1 flash.

From what you said the torque increase at ~4000 rpm is due to the resonant intake manifold. It seems as the power loss in the 3000 to 4000 rpm range coincides with the cam advance. I'll try to determine if the solenoid is activated while driving.

What is the short tube on the actuator of the resonant flap, next to the connector? Is it an adjusting screw, and if so, how is it adjusted? This seems to be a very unknown part of the engine as it does not appear in any manuals. Not even in the factory CD's that I have. Perhaps it never causes any trouble.