head shake when idle,what can be? on 2004 Cadillac SRX

in the morning when the car its cold,and its getting warmer it does less,i change the spark plug and does less often.but still does the head shake

Asked by for the 2004 Cadillac SRX
Mine is doing it too. They are telling me it is the Catalytic Converters.
1 more answer
Have your mechanic check for a leaking head gasket. I had thqat problem in my V8 and the dealership put in anti leak tabs into the cooling system and that did the trick. They do not tell you but, you should have the cooling system checked every 30k or so and get the tabs. it helps to seal the system and, it recommended by the factory.