Engine jerks at certain speed

Hello, I’m writing a problem for an acquaintance here, he bought an Audi A2 from 2000 it’s a 1.4 75 ps gasoline. He has the problem that the engine is shaking extremely at 50-55 kmh and a certain speed and the whole car is shaking, but as soon as you give it a little bit of gas everything is gone again. In the stand he runs completely quiet and without a dropout. The error memory is also empty Had maybe someone this problem or a similar problem, so that he Maybe knows where he has to start to solve the problem and he drives back reasonable without shivering stuttering greetings