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

Huh??? You've lost me. Light bulbs affect the cruise control? What bulbs are these that I should check?

Now that I think about it, the only bulbs that have been replaced are brake light bulbs. And, if I remember, (it's been awhile), I did replace one of them with an auto store purchased bulb.

ONLY the BRAKE bulbs place a role in the cruise control circuitry. The ones in the tail lights. That's it.

Keep in mind, it is highly unlikely they are the ones causing your problem, since your cruise control works some times. 15 out of 20 times the problem is the cruise control amplifier.

Do a search, you will find everything you need.

This is a very old topic and there are many discussions in the forum.

Good Luck!!

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

three soultions to look into
rear brake bulbs
All fresh fuses
cruise control amp solder connections resoldered.