If the shaft on the butterfly is worn it may stick, if the return spring is weak it will also not return to idle position. Idle above ~1000 rpm will cause the fluctuation you are describing. If the throttle shaft is not returning to it's rest (idle) position, the idle will be above 1000rpm causing the flucuation. I believe your year has a dashpot (I think that is what it is called) that softens the return of the throttle shaft at idle (below & in front of throttle linkage), that can also be out of adjustment & possibly cause the problem. I am sure others will have other causes that I haven't thought of.
