my 95 w124 has this problem as well. It's good as long as it's running then I stop the car and come back and warm to hot air came out. Freon was checked and good. Then about 3 years ago...an indy mercedes mechanic noticed the compessor was stuck, so in a pinch he gave it a whack and presto....it spun. He then carefully sprayed to penetrating oil on the outside of it. Well...it just got stuck again tonight (MN was 96 degrees and muggy today). Ice cold on my 25 min commute, then I ran into costco and hot air was blowing out when I restarted my car. I went under the hood and saw the center part on the compressor was not spinning again...so repeated the whack(light tap)...used a long 18 inch flat screw driver and gave it a tap....then it spun again. I carefully dripped some penetrating oil while avoiding getting belt while the compressor was running and when the car was off. I just tested it the past 2 hour and it works like normal again. I report back if it goes belly up.
Anyone know how tough or approximate cost to change the compressor alone?