Check the throttle linkage for ease of movement. Also check the throttle plate (the rod that goes straight down near the front of the air flow sesor plate housing). These cars can get enough crud around the throttle plate to make the plate stick. When the cruise goes from closed throttle to open throttle, it has to overcome this resistance to movement. Then when the throttle "snaps" open, its open too far...so the cruise backs off causing a surge.
ASE Master Tech
Mercedes Tech 19 Years