2000 Renault Laguna car shaking by standing still

Hi,

just bought this car and wondering if i should be worried about:

The weehl and the geear-switch (basicly the whole car) are shaking if the enginer is on, but car is standing still, however if i am driving than its all ok, it looks almost as the engine is not getting enough fuel if i dont push the gas.

Thanks in Advance,

Istvan