Radio Power Fluctuations in 206?

I have a Goodmans CD/Radio that works fine when the engine is off, but as soon as I start the engine the power fluctuates causing the display to dim and brighten repeatedly. If I play a CD (with the engine on) then it takes forever to 'search' for the tracks, and when it does find them and start to play the first track it'll cut out in seconds and start searching again. But as I say, it works absolutely perfectly without the engine running. Any ideas?