I have a manual 2018 A4 S-line 1.4 TFSI Saloon.

Last year it came back from a service and immediately exhibited this problem.

Audi recommended the spark plugs be replaced. They did this but it made no difference. 

The symptoms:

When cruising at a constant speed on the flat, the point of minimal load on the engine, the engine judders. It can feel quite strong at times.

Although this symptom is intermittent it is quite frequent.

If, while it's happening, I drop my speed a fraction the juddering stops during the deceleration but might start up again after a while at the new speed.

If, while it's happening, I press the accelerator, the juddering becomes more erratic and the speed remains constant for a moment before pulling itself out of this state.

It feels like the engine recognises this condition of constant revs with little load and shuts down a little.

Any help would be appreciated

Something that might be relevant:

There are no errors shown on the dashboard.

Using a cheap OBD2 reader and the Torque Pro Android app, I also see no errors reported.

Terry.

If the sophisticated reader at Audi does not find anything, your cheap reader has no chance. All I can suggest is that you find an alternative garage to test it. We often recommend independant VAG specialists over main dealer. An internet search should find those near you.