Power loss, dyno graph.

[C250CDI]

Hi everyone!
My car w204 C250CDI (2009) biturbo, 204HP, 500Nm manual, has an intermittent irregular acceleration. I can feel the lack/lost of power in one from ten accelerations at around 1800rpm, 2800rpm and 3800rpm as you can see in the dyno graph.
I have the original soft (not remaped).
No errors stored in ECU (checked with STAR c4).
All 3 turbo vacuum cells work well.
All 4 injectors work in normal limits.
No (apparent) vacuum leaks.
Apparent all the air hoses are fine.
What may cause the lost of power?
Where and what should I check first?
-The turbo?
-air intake?
-fuel injection?

PS I went to a MB dealer but they have not found any problem at my car. The engine work in normal parameter, they said.

I appreciate your help. Thanks in advance!

 

[estate-agent]

How many times was the car run up to produce this graph? If it is intermittent and on other occasions the graph and power delivery is more linier then I'd be surprised if the issue can be found. To be fair that power delivery is still pretty good but if it is noticeable when driving on the road I would imagine there is a little bit of boost pressure drop off at those rpm points or a very minor variation in its fuelling or timing that is undetected by any of the cars sensors. Answer to this will likely be a remap if it really does bother you. Bonus is it'll be quicker with your remap too.

 

[C250CDI]

Thank for your reply, estate-agent.
On the dynamometer the car has 3 attempts; all 3 graphs were almost the same. I can feel the power loss especially on full throttle but not all the time. It is "an on and off" problem... I wonder why the lack of power occurs at ~ 1800rpm, 2800 rpm and 3800 rpm? I plan to change the Turbo Boost VACUUM valve first(it is not so expensive). Then to check at a MB dealer if there is a soft update available for my ecu. Any other suggestions?