Not running on all cylinders

Now I have my first problem, not even a week of the car and already it spins. The following: Since this morning, it sounds like my Omega 2.5 V6, Bj 98, Automatic, Autogas does not run on all cylinders. He does not really take gas and shakes himself even a little when I stand at the traffic light or something. Consumption at the level 1.7 – 1.8Ltr, before it was 1.2 – 1.3Ltr. He does this both in gas operation and in petrol operation. n 500 – 800 revolutions. Who can tell me what it could be Gruss Holger