Electrical problem: radio/200Tdi/lights/earth (?)

The CD/digital radio in my 1991 200Tdi has recently developed a fault, which is apparently (according to internet searches) linked to a poor earth in the light system.

That is, it will work fine for a while (5 miles or so) with the headlights on, then turn itself off. Then back on. Then off again. Repeat until you get bored of it. It continues after I turn the lights off, however.

My questions are, is it really likely to be linked to the light earths, and if so, where would I find the offending part in the car?

I'd run a new earth to the radio to check. Just a temporary lead from the battery. If that sorts the problem on a test drive, then start by remaking the earths behind the dash/anywhere you can get at them.

Thanks, I'll give it a try.