Problems with the ground in the antenna system can cause this. This can be in the connector at the radio end, or in the mount at the antenna end. It can also be in the head unit itself, (an open ground between the AM antenna tuning circuit and the antenna connector ground). This can be diagnosed with a head unit from another Crossfire; if you take the unit from the other car and it does not do it in YOUR car, your head unit is to blame.

As to the antenna:
I' have never taken my antenna mount apart, never had the problem myself, and I've never looked closely at how the antenna mount goes together. Sorry I can't be of more help.

I would try swapping head units with another Crossfire if anyone has one close to you. This is an EASY diagnostic trick, at least you'll know if the head unit is OK, then you can move on from there.

Is this a Coupe or Roadster?