Well, the easiest and certainly the cheapest are light bulbs. Make sure you're using the correct, European bulbs. Even though a replacement GE or Sylvania bulb may have the indicated cross-reference code number, they draw slightly different amounts of current, and that's enough to affect the very sensitive cruise control in precisely the way that you describe. Intermittently, unpredictably, sometimes immediately, sometimes not for 30 miles. BTDT.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't