i just had the same prob with my 89 5.0,i went the rout of idle air valve & even the tps,but in the end it was a nasty mass air elements. it was acting the same as you describe. it was driven me nuts. look at the elements(lil coils) if they are intact then try cleaning them with rubbing alcohol not carb cleaner.they should be a normal metal color not black as i found out.
providing yours is fuel infected not carbed,
idle valve i foud out wont do that,it will give a ****ty idle or a lil low,then high,but not that high.good luck
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