I've got a 1997 Lincoln Town Car, Cartier Series. I notice that when idling in the Drive Gear, the car sometimes surges ahead, occasionally with force a bit stronger than the force with which I've got the brakes holding it back. And sometimes, the engine dips really low, almost to the point where it seems like it's going to stall, which thankfully it doesn't.
One other pattern: the car only does this when I've got the heater or A/C on.
What could this be and how might I repair it? I took it to Midas and they adjusted the idle, which helped but didn't ultimately solve the problem. TIA.
-The Derfer