It's a little hard to diagnose without specilist equipment - it could be that the radio is overly EMI sensitive due to poor design, it could be faulty, you could have a faulty alternator.

Is it a standard 6006? If so I'd be looking elsewhere than the radio as this isn't a 'feature'! You could try using a clip-on ferrite core on the antenna lead, but I think it's going to be alternator related.

HTH
Chris