Have a '82 300SD w/64k ... problem is the heat temps. It's winter - snow & cold now and heat is a necessity! While driving the heat will be fine warm and toasty then suddenly blow cool. If you stop and turn the car ignition off and restart the car to drive again , the heat returns warm once again. This may happen every other day or so over several hours of driving. The thermostat has been replaced - what's next ? Thanks in advance for your help.