Got the shakes

[pandamonium]

Since I've changed the engine mounts and gearbox mounts on the Avenger, it has developed the shakes. When at idle, the engine shakes and sends vibrations through the car. I reckon the problem was there before, its just the old soggy mounts were absorbing most of it previously. Any ideas? Just an old car thing? Or something more serious?
Thanks in advance for your help!