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Idle problems

My car ('87 5000CS TQ) is having idle problems. The car normally idles
at 800 RPM but lately, the revs have begun to drop to 400 or 600 and
the car sometimes stalls. This is especially the case when the A/C is
on and the clutch is depressed (its a manual) when the car approaches an
intersection. The revs drop low suddenly and the idle stabilization system
is unable to recover. The car runs fine under acceleration. Any one have
any ideas on what could be wrong?

Here's what I have checked so far:
When the connector to the idle stabilization valve is disconnected, the
car stalls (implying that the valve is working?). The $*@#^% Bentley
manual is useless as far as this is concerned. It has a section on the
idle stabilization system for CIS (continuous injection system, which is
what my car has vs. the CIS-III or the CIS-E systems) but this covers only
the 84-85 turbos, not the 86-88 turbos. The section on the 86-88
turbos doesn't cover the checking procedures at all, only how to set
the CO and OXS duty cyles, etc. So I don't know if the specs for the 84-85
turbos hold true for my car. For example, the manual says there should be
12 volts at the idle stabilizer control valve connector when ignition
is turned on. I don't get 12 volts on my car but the car will stall if the
connector is removed. I am seriously considering calling up Bentley
and giving them a piece of my mind.

Anyway, back to the problem...
The throttle idle position switch checks out OK.
I threw a bottle of Chevron Techron FI cleaner in the gas tank; didn't help.
The spark plugs are new. The air filter (K&N) was put in only about 12K miles
back. I figured that the A/C control unit may not be sending the signal to
increase idle speed when A/C is switched on. I need to check this, but the
problem is there, though to a lesser extent, even when A/C is not on.
I increased the RPM using the adj. screw in the throttle body, helped
slightly but the problem remains.

What else should I check? Any help will be appreciated. Thanks.