RPM fluctuates due to timing changes as the point at which the fuel is reaching it's peak expansion varies drastically with just a couple degrees. Tweak it too much either way to compensate for an idle issue and you put other components at risk when the loads increase.
Try cleaning or replace the ISC. Make sure your spark plugs are not fouled, plug wires are firmly connected, vacuum lines all run properly, and make sure the mechanical timing is 100% correct.
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