Just had a similar problem with the CCU on my '85 500SL. Only it was the CCU itself. One way to find out if it's the box is to get someone to put their hand on it while it's acting up. (Mine mounts on the firewall behind the glove box on the passenger side.) You can definitely feel the relay(s) sticking in the box.

Pulled the box and opened it up. Cleaned some contacts and relays that were actually dirty, but it didn't fix the problem. One incidental (?) side effect of this was that when stopped and it acted up the RPM would surge from 475 up to about 800. Don't know what was causing that.

The really neat part of this is my CCU #000 822 12 03 was priced at $258.00 through the MB dealership when I first started looking for the part. Once I figured out that I'd have to replace the box I started inquiring of places like FastLane to see what I could get it for through them. Lo and behold!!! MB had raised the price on the CCU to $1,574.00. No kidding!!!!!!!!!!!! FastLane did offer it to me for about $1,200.

That was still a big chunk of change so I did a bit of inquiring and found another unit with a different part number that I bought for $125.00. It's missing one pin on the side that has four pins. Put it on yesterday and I've got heat, air, and a fan. Don't know what I'm missing and probably won't find out without a schematic.

FYI, I also went ahead and replaced the Overload Protection Relay since I'd had to replace the idle control valve, idle control unit, and now this. She's running like a top now.

Cheers!

Don