I'd pull up to a traffic light and my CCU would go bzzzzzzzzzttt and the idle would surge two or three hundred rpm at the same time. I would also hear the contacts sticking at times while driving, but couldn't detect an rpm surge then. It was only when I was stopped that the surge was noticeable. Figured out it was the CCU by pulling the glove box and had a buddy ride with me to lunch and put his hand on the various boxes mounted to the firewall. The CCU turned out to be the culprit. This CCU is NOT the Heater and A/C controls in the dash, but is a "black box" that mounts to the firewall. Believe some folks might tend to confuse the two.
Interesting part about this was when I first began tracking the problem and checked the price at the MB dealership it was something like $154.00. Figured what the heck and went to order it. They'd had a price increase to the tune of $1,500.00+. Started searching the internet for one and found a guy at TriStarpete.com who came up with a "suitable substitute" for $125.00. It had three prongs on the right side vs the four on mine, but seemed to work. Turned out that what I lost by not having the fourth prong was the a/c cycling. That's why I decided to try it again after replacing the voltage regulator. Glad I did.
Hope this helps.
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000