Most techs have an idle air control tool......you disconnect the harness, attach the tool, and you can increase/decrease the opening of the passageway, via the pintle....
Or you look at the IAC counts on a scan tool while the engine is running....if idle is high, but counts are low(under 15), then you have a vac leak......if the counts are high, then the IAC is being commanded high by the PCM.....
One thing I have found, is an intermittent connection in the IAC harness....either the female connectors are spread/loose, or you could have a broken wire inside the insulation making an intermittent contact(usually breaks within 5" of the harness end).....
Sounds like you may have some wiring problems, with your cluster acting up too....
Another thing to try, get the car to normal operating temp...shut vehicle down....turn key to on, don't crank....disconnect the IAC harness...now start car and see what idle does....