Interference. Are you outisde? By a highway? I've seen trucker's CB's mess w/ cars before.
Is your antena up on your car, or did you try some internal antena mod? If so, get it back where it belongs.
Are you pointing your Tx at the car? Don't . . . there is usually a dead spot right out the tip of the antena on the Tx.
I guess that's all I've got
