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An idle speed which rises & falls.....especially at traffic lights when the engine is fully-warm.....is nearly always due to a faulty TPS (throttle position sensor).
If I remember correctly, on the standard car there is an adjustment screw which opens the throttle butterfly disc a crack to set the revs.. This may be missing on your bored-out throttle body, leaving you no choice other than to make the adjustment by means of the throttle cable adjuster.
If you have high steady revs. when the butterfly is fully closed, then either the disc does not fully block the new throttle body, or there is a fault or split hose on the idle control actuator.
Nick. :)
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