E320 M104 power loss

Hi
I need some guidance.
My E320 W104 1995, runs fine, except for a decrease in torque from about 3000 rpm. Currently the torque increases from about 4000 rpm, and at lower revs it is pretty flat compared to what it use to be.

Can it be the resonant flap in the inlet manifold ?
How is this flap actuated, electrically or electro-pneumatically. I do not see a vacuum line to the actuator.
Can it be cleaned or serviced?
At what speed does the flap change position?

Can it be the cam advance?
The solenoid is alive as can 'audibly' move the piston when externally actuated from the battery.
The signal to the solenoid is high between about 3000 and 4000 rpm

Can the MAF sensor function partially and cause this problem while the engine runs normally in other regions.
Fuel consumption increased from 10l/100km to just under 11l/100 km since the problem started.

Wiring harness is new.

I could not locate any meaningful fault codes from pin 8 in the 16 pin connector. It gave 36 flashes and 42 flashes. These are unknown to me. I have reset the codes and will check after running a few days.

Any ideas would be appreciated !
Thank You
Gerritt Visser