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Sure sounds to me like a bad CIS motor (aka the idle air valve), a bad or misadjusted throttle switch, or a bad connection or bad wiring to either the switch or the idle air valve.
I had this problem on my '86 245 and my '82 245 turbo.
The "throttle plate" is the round plate in the throttle body, or throat. It rotates about a diametric axis to allow more or less air into the engine. It's a manual air valve (controlled by your right foot). When it's closed, almost no air goes by it -- idle speed is controlled by a bypass route (around the throttle body) via the CIS motor (idle air valve). As you step on the gas pedal, the throttle opens to allow large amounts of air into the engine.
The idle air valve is controlled electronically -- when the computer sees the idle dropping, it opens this electric valve to allow a bit more air, which raises the idle. Likewise, if the idle climbs, the computer closes the air valve slightly to reduce idle speed.
The throttle switch tells the computer that the throttle is at idle position and that the idle air valve should take over control. If the switch doesn't "click," then the computer never tries to control idle.
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Don Foster (near Cape Cod, MA)
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