I am having a problem with the idle on my 740 turbo that forces me to drive with both feet (it is an automatic) the way I used to drive my long-gone 1978 plymouth volare. Whenever I take my foot off the gas, the idle drops to 500 rpm and then about 15 seconds later, the car stalls. I have checked all of the vacuum lines I can find and they all seem ok. I think it could be a vacuum leak in the heating/cooling system controls, but I can't find the lines that feed into them from the engine compartment. I disconnected and plugged the line that goes through the firewall from the top of the valve cover, but that turned out to be the line for the turbo gauge. The car runs great otherwise, so I assume I can rule out fuel pump, ignition wires, etc. BTW, the spark plugs, distributor cap and rotor are all new. Any suggestions?