What causes my Chevrolet Silverado to erraticaly low?

[the light of linux]

It's a 1988 Chevrolet C1500, 350 V8, Turbo 400 tranny. Basically, it runs fine for a while, then when idling in park or neutral, the idle drops from 850 to 500 - 600 RPM. My code reader isn't throwing any codes, so any suggestions are greatly appriciated. Thanks.
No carb since it has TBI. Has new plugs, cap, and rotor. Plug wire test good on my Ohm meter.Fuel filters are new since the truck doesn't have more than 15,000 miles on it. Also, I don't run additives, because most just cover up problems.

 

[imjames]

Replace your Idle Air Control Valve. AC Delco P/N 25527077.
$48.79 on RockAuto.com.
Autozone carries a Duralast for $44.99. The part number is IAC92174A.

The idle air control valve regulates the amount of air that is allowed to bypass when the throttle body is closed/closing. If this valve is faulty or sticking, it will not be able to properly open or close, causing the engine to stumble due to the lack of airflow to the engine. This can also cause hard starts and stalling on deceleration. The throttle plate snaps shut, the IAC does not compensate and the engine receives very little or no air, causing a stumble. Same thing at idle.

I've replaced many of these, they are prone to failure. The good thing is that they are relatively easy to replace.

Testing:
Disconnect the electrical connector on the IAC (the IAC is located on the throttle body, passenger side. It sticks straight out and is located in front of the Throttle Positioning Sensor.)

Use a volt/ohm meter to check resistance between the light green/white and the light green/black wires.
Check the resistance between the light blue/white and light blue/black wires.
The resistance should be infinte. This means that your ohm meter should show a 1 and a period with no other numbers.

If this is not the case, the IAC is very likely bad.