I had a similar problem on the 300SE sometime about 2 years ago. The problem was solved when I insisted they check the fuse in the box. Since an average fuse cost about US$0.05 each, I had them change ALL the fuses, and voilaaah ... the c.c. worked again.
However, this morning as I was cruising on control and going down a hill gradient, I noticed some minor jerks. Perhaps its the system trying to maintain the pre-set speed while going down the slope?
I certainly hope it is.
And I certainly hope its your fuse too ..