On my 89 420SEL with 150K miles, the AC comes and goes, the local guy says the freon is OK and it's the fan clutch, I read on this board that the thermostat might be bad causing the fan clutch not to work.
The electric cooling fan comes on with in 30 seconds after the engine is started cold, but the engine does not run hot, the temp gauge is always normal. The car is run very little, 100 miles / two weeks.

Fan clutch is $250, thermostat is $30.