No I think they are right. Sounds like your mixture is too rich. There is less oxygen at higher altitudes. But 3000ft elevation change shouldn't affect it much unless yours was off to begin with. Your idle control valve is probably not opening up properly either. Take it to a Mercedes garage and have them put on an exhaust gas analyser and adjust the mixture screw on the fuel distributor. Don't try this at home unless you keep track of your changes. Very subtle adjustments and each adjustment takes a few minutes to take affect.