Why after market radio looses FM signal

Hey,

With due respect….

I have a question to all of the technical person here and also to those who have experienced same issues as i am facing these days and i have noticed it in different cars.

I have American Infiniti Qx4 2000 model….runs well….i am using it in UAE (Dubai).
Problem i am facing is : whenever i drive my car looses FM signal….as fast as i drive signal distort and i do not listen FM clearly.
Other channels are also weak. When i was installing after market car stereo – freshly it worked well but soon channels disappeared.

What can i do…? i have installed new antena as well but things are same – only very little improvement that i can only see.

Can any one guide me?