How did the tech fill the freon without a working compressor ? I dont know about your car but in my 260E and almost every other car, you have a little glass window on or near the drier that tells you if the compressor is working at all. You are likely to see some vapours in that window or bubbles if you are low on freon. Sometimes, the main belt that runs the compressor is not tight enough and so the compressor does not work effectively when engine rpm drops. Ckeck belt tension also. As far as I can tell, it is highly unlikely that you have a intermittent compressor, either it works or it does not. If you see something happening in the window then the next thing to worry about is the condensor fan. Is your condensor fan working ? if it is not working or if it is wired wrong (i.e. if it is blowing air away from the front of the car rather than towards the engine) then when the speed is low the compressed freon will not be cooled enough before it goes to the evaporator and you will not get cold air. You can check the air flow by holding a small piece of your newspaper in the front, the paper has to be sucked towards the condensor otherwise it is not wired correctly. Also check your engine fan, make sure it is working adequately. Is your car running hot ?
If you cant check these things yourself, take it to a reputed MB tech not Midas OR you can take it to a repair facility that sells A/C parts only, i.e. just A/C repair facility. I dont think changing from R12 was necessary and neither was a new evaporator. The only thing that can go wrong with an evaporator is a leak and it looks like you did not have that. Mercedes benzs have very easy checkup methods provided the tech knows about it. I would look up yellow pages and pick a tech that has been in business the longest.
Saumil S. Patel