[Gelbster]

The Variocam on my 2001 S actuates progressively and reaches full advance before 3000 rpm.But it must be a warm engine.
In my limited experience ,I have noticed other Boxsters (inc mine) have more deviation on one bank than the other. Weaker/worn chain tensioner on one side??
In theory the solenoid can fail intermittently(Connections?) or gradually and the oilways in the actuator can plug with debris.That may be your issue?
You might want to confirm the basic timing of the cams is correct (green plugs) ?
If you need a more comprehensive discussion, it was covered in detail on Renntech recently.But try the simple diagnostics first in my experience with Variocam.There is an Ohm test, a click test, and others that involve fabricating test harnesses .Do every test you can find before deciding to remove cam covers ! For example do a road test & use a passenger to monitor with Durametric live data for Angles in Actual Values on both Banks simultaneously.You can log the results.