Warm engine idle fluctuation

[RonPellerin at aol.com]

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My 1988 90Q idle is about 1200 RPM when fully warmed up. It then starts to
fluctuate between 1220 and 750 every two seconds. It only remains at 750 for
an instant. I've changed the idle stabilization valve and squirted WD40
around vacuum lines with no success. I have been told this could be the O2
sensor or air control potentiometer. I also wonder about the temperature
multi-switch on the radiator hose neck. When I disconnect the idle
stabilization valve and replace the broken vacuum connection, the idle drops
to 750, leading me to believe the ECU is directing the fluctuation by some
malfunctioning sensor input. My temperature gauge is also erratic. I have
been told that there is a vltage regulator located in the gauge panel which
controls the reference vltage for both the water temp gauge and the fuel
gauge, which by the way, also does not read full when it actually is full.
Since the gauge is nothing more than an milliammeter, with a calibrated
potentiometer float, I would assume a regulated voltage lower than what it
should be would give a needle deflection of less than it should for a full
tank. Any comments would be greatly appreciated. Thanks in advance, Ron
Pellerin