Why does my Sav Nav loose its satelite signal when i turn on my car stereo?

Hi, I don't know if anyone can help me here. I have a Garmin Nuvi 265W. Quite often i loose my satellite reception, it was only tonight that i noticed that each time I turn my stereo on I loose my satellite reception, as soon as I turn the stereo off the reception comes back strong again. I'm using a JVC stereo which is running a Jenson amp in the back of the car.

Does anyone have any ideas what the problem could be?

Thank in advance.