This issue has been discussed at length on the BenzWorld W210 forum - another good resource for that chassis. If you're lucky, you can take off the knob and spray electronics cleaner down into the hole behind it, it may help clean out a little gunk. It helped mine, though it didn't fix it. But eventually this doesn't work anymore, and the only real fix is to remove the radio to replace it, or have it reconditioned by Becker.

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Charlie

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'66 VW 1300 96K miles
'97 E300D 239K miles
'85 300D 203K miles (sold Sep 2012)