Engine Misfire with GRA (ALH) - Troubleshooting

I have been noticing the following for a year now.

When I'm driving at around 70-80 km/h in 5th or 4th gear with the cruise control engaged, I notice that the engine slightly accelerates intermittently. This is on a flat road.

'What could be the cause of this...? I never experience this 'jerking' at any other speed ranges. Also, it doesn't happen when I maintain a constant speed without using the GRA (presumably an acceleration assist system).'




Car:
Golf IV TDI - ALH engine, 66kW - no chip tuning, no biodiesel, EGR is currently disabled, filter was recently cleaned (K&N replacement filter), MAF sensor seems okay (running in 5th gear at 4200 rpm, speedometer almost 190 km/h).




Sorry - one more question:
Sometimes, on the highway, it feels like I'm experiencing different levels of power. Sometimes the TDI pulls well all the way up to reaching its maximum speed, but shortly after, it takes a little longer to reach the maximum speed. It feels like a few horsepower are suddenly missing . Could it be due to a sticking VTG (Variable Turbine Geometry)?