bad engine mounts.
the engine would rock around and shake in idle. It would be more noticable if you put the car in neutral and revved the engine.
mounts do go bad but the shaking is more consistant where as the problems above are indicated buy loss of power, hesitation, and a feeling that the engine is choking for gas.
best person to speak too is your mechanic... they could tell you the problem right away. It's not something that can be diagnosed online.
as others has stated could be a lot of things that is causing this problem
