wow, this is an old thread, but I'm having exactly the same problems at this moment! Last week I had my cambelt changed (including all pullies and stuff). Since then, the engine vibrates, especially when idling. Also, when driving with a constant speed, it feels like the engine isn't running perfectly smoothly, but ever so slightly.
The guys at the garage checked everything, assured me the timing is correct, also the air flow meter is functioning properly, they even cleaned something that I don't know the English name of, but the problem still exists. Also, I think they replaced the top mounting bush, if I understand correctly what's meant by this.
They recommended that I change the spark plugs. I will do that, but given the information in this thread, that might not be the problem. Also, they (and myself, and a friend of mine) think it feels like one of the 6 cylinders isn't working properly.
Maybe someone can shed some light on this, since I feel this thread hasn't quite given the answer to the initial problem.