Ed,

It sounds like the refrigerant charge could be low. Higher engine speeds mean higher compressor speeds, ie: more coolant circulating. Also higher road speeds means more air moving through the condenser, leading to better heat transfer, (and cooler air flowing from the dash vents).

Conversely, low engine speeds and low air flow through the condenser, (ie: car parked and idling), means less coolant circulating and less heat transfer, (warmer air flowing from the dash vents). I would try a recharge provided that your aux. fan is working.

Good luck,

Steve
2002 W210, 53K