I have a 2000 CLK55. I've been Having this strange problem with the idle hunting when the AC compressor disengages. What happens is the AC compressor engages and the idle slightly changes. When it disengages, the idle dips and starts hunting. It almost feels as though the car is about to die. If I blip the throttle, the idle will settle down. However, if I leave it, it gradually sorts itself out until it has stabilised over 10 to 20 seconds (I haven't timed it). The scenario repeats itself every time the AC compressor disengages. This is only happens at idle, e.g. when I come to a stop at the lights or when stuck in traffic. It's more pronounced on a hot day when the AC compressor comes on and off more often.

I've given it a service today, not including plugs or fuel filter - they will be done soon though. I also changed engine and gearbox mounts, and cleaned the throttle body and maf. I made sure the airbox was seated correctly. There's still no change after doing that.

This sounds like an idle control valve issue to me. I don't believe it's a vac leak, unless it so small that the idle control valve is able to compensate for it.

Has anyone got any thoughts or experience of this?

Sounds like unmetered air (leak). What "service" did you do recently? Sounds like something got bumped, cracked or knocked loose.

Check all sensor connectors and air pathways, including PCV, evap and EGR if present.

It does sound like a vac leak, however it only occurs when the AC is on, which to me is strange. I'm not dismissing the idea, but in my experience a vac leak would be apparent at all times.

I changed oil and filter this weekend. I don't go by the book and its 'a service' and 'b service' plans, I do what I deem necessary.
This issue has been going on for a couple of months, and gradually getting worse. There has been no recent work to the car before it started behaving this way, so it's unlikely that something has been knocked.
I've checked vac hoses and found nothing. If it's a pinhole then it'll be hard to spot.

Ok a bit more news...

I've been trying various things to identify the cause and effect. It now appears that it's happening even with the AC off. It takes a 10-20 seconds of being stuck in traffic, then the engine will hunt and feel like it's misfiring. It eventually clears itself.
I tried to blip the accelerator whilst it was happening, and it was sluggish to react. Pretty much like a misfire on multiple cylinders.

My thinking now leads me to believe the issue is related to heat soak. What is the most likely component to be affected in this way? MAF? Inlet air temp? Coolant temp? Something else?

I've had it hooked up to a scanner. Live readings indicated MAF is knackered. Got a new one ordered. If you don't hear back from me, assume all is well...

Besides the leaky PCV, if your two upstream O2/AF Sensors1 are over 100kMi, you'd benefit from replacing them in responsiveness and gas savings. You can leave the downstream sensor2 alone.

These wideband sensors are super closely monitored by ECU to prevent smog... yet they age and react more slowly and each bank gets a bit out of synch from each other.

They directly affects the AF mixture by driving the fuel Trims corrections.
So long they are reading and heating: you wont get a CEL, only poor mixture.

Thanks. I'll bear that in mind, but I'm only spending on what's necessary. The car's value means it's not worth putting too much into it.

I've replaced the MAF and seems ok so far, howover after a little while of idling, it does get a touch rougher. But not significantly.
I did spot the vac line from the egr to the manifold was split, so I've fixed that too.
We'll see how it goes over the next week.