I've experienced a similar problem with a W203 C240- rough idle, worsens with A/C on, and this condition deteriorated with time (~1 year) to include a very apparent loss of power.

The car's been to the dealership a number of times, they've been good at goodwill-ing some service, but the problem is still there. They've replaced:

O2 sensor
plug wire sets + coil packs
plugs ($100 in just plugs!?!)
MAF
Air filter

Additionally, they've performed a compression test, computer diagnostic, adaptation, code scan...and still have not been able to fully resolve the problem.

All the work has smoothed out the idle somewhat, but loss of power is still there. It has been recommended that I try swapping the ECU, or that a glitch may exist in something emissions related. But that's an expensive gamble that may or may not fix the problem.

Sorry for the rant, hope someone figures out why this is happening.