Actually, I have a similar problem in an '89 190E 2.6. I've had the car a little less than two years. It has always had a subtle trailing-throttle "hunt" condition when in cruise. This has not been too objectionable. Lately, though, the cruise will exhibit a much more dramatic cut-out, cut-in behavior that doesn't seem to be tied to any particular throttle position and isn't consistent (doesn't happen all the time). A couple of times this has been so extreme that the cruise has actually turned itself off.
I'll try the throttle lube, but have been considering a full resolder of the cruise amp as well. If anyone's had this experience would appreciate some comments. Thanks
89 190E 2.6