Radio Interference

[LesD]

Hi,
Can anyone help on this one? I'm constantly experiencing interference coming through the speakers whenever I use the radio/CD/Cassette in my XK8 convertible. It sounds like the unit is not supressed. The problem is intermittent, and I can sometimes clear it by pressing the radio into the surround, but as soon as you go over a bump in the road, the interference returns. The problem has been investigated several times by my local dealership, who replaced the radio under the warranty. but to no avail. To me, it sounds like an earthing problem.....any ideas???