OOOPS Red face, red face.:z: <img src="/forums/images/graemlins/zombie.gif" alt="" /> I clicked on "edit" as moderator on your post above instead of "reply" sorry.. meaning to post the following answer.....
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You NAPA guy would be correct if your Toyota used a hot wire MAF sensor, but I believe your '90 is a moving air vane type MAF sensor. K&N Filters can cause problems with the hot wire sensors, but the symptom of that is not what you have..

Your problem symptoms indicate that your throttle position switch is not electrically closing the contacts that indicate to the Engine ECU that the throttle is mechanically at idle..

Without this circuit going closed, the ECU will not go into the mode to regulate the idle speed or, some Toyota ECUs will literally oscillate the speed from 800 - 1300 rpm as you describe.. by out of control running the idle air control valve back and forth..

Many times this problem is caused by people mistakenly adjusting what looks like the idle speed screw on the throttle body, thinking that this will change the idle speed (NOT! on a ECU controlled Idle Air Valve controlled system)...

Wear on the throttle body shaft can also cause this problem to start happening, and symptomatic of the cause when "kicking" the throttle causes the problem to sometimes go away.

What looks like an idle speed adjustment screw is used to set a baseline clearance of the throttle body butterfly plate off the throttle wall by a spec'd amount in the procedure.. Then all other TPS adjustments are referenced to this. You need to go through the TPS switch adjustment process using the Toyota service procedure for this... If that don't work, you may need to replace the TPS switch..