Audi A6 2.5 TDI: Specs & Top Speed (200 km/h)

Hello everyone,
I'm experiencing the following problems with my Audi A6 2.5 TDI (engine code: AEL, 140 hp). It only reaches a speed of up to 200 km/h.
'It suddenly drops off at around 202 km/h and then settles down to 200 km/h. Also, when I'm in 6th gear at 130 km/h and give it full throttle, there's a slight jerking sensation. If I apply the gas more gently, you don't notice anything.'
Reading the error memory revealed the following:
Error code 550: Spray start control - difference between setpoint and actual value.
So far, I have been able to rule out the following:
Fuel supply system, complete, in good working order.
New genuine AUDI diesel filter.
No air in the fuel system.
Needle movement sensor check performed at 3000 RPM; OK.
The ignition timing was dynamically adjusted using a VAG diagnostic tool. It also starts up perfectly and only produces smoke under heavy acceleration.
I was told by Bosch that the start of injection should always be adjusted using a timing light.
Injection timing valve: It was activated and controlled using the tester. However, I'm not entirely sure if the valve is actually 100% functional.
Fuel injection pump stroke blocked: I can't imagine that being the issue, since everything seems fine except for the maximum speed (acceleration is good up to higher RPMs).
Okay, now I hope someone can help me with this.
Thank you in advance.
Stephan.

Hey everyone, does anyone have any ideas?

Sure. But why don't you come up with the idea yourself to check the control loop for the start of injection? What is the duty cycle of the solenoid valve used to adjust the start of injection in the relevant speed/load range?
What is the difference between target values and actual values?

How can I help you if you cannot understand the term "spritzbeginn regeldifferenz"?