'95 E320 A/C switches to heat after cooling
 

The A/C in my '95 E320 goes into heater mode (or humidify) mode after the car has cooled to the desired temp. The only way to get the back to cool is to turn off the car and crank it up again. There is then no delay in the cooling again, although it will stop again when it has done it's job.
If the thermostat is turned all the way to cool when the car is first started, things stay cool and sometimes even get too cold, without any interuption.
Any help? :o