I agree with the belt tension comment. Not sure of the arrangement of the belt on a 300E, but my 88' 190E has a single serpentine belt that drives all the auxiliaries. There is a spring loaded belt tensioner pulley that maintains proper tension on the belt. For me, the tensioner spring had weakend over the years and was not applying enough tension to keep the A/C compressor pulley fully engaged except at highway speeds. I replaced the tensioner and the tensioner damper (a small shock absorber attached to the tensioner) and the A/C works fine now.