I am having a problem with my 87 ford f-150. The engine revs up and down randomly at idle and sometimes stalls, especially when I pull into my driveway, right before I stop. Half the time its ok the other half of the time it revs up and down. I suspect a vacuum leak but I cant find one anywhere and i would think a vacuum leak would be a constant problem not an intermittent one. Is there anything else it could be?