something is changing your timing. like the one guy said weak springs or points or something. my vw did this. I could set the cruise at any speed, like say 65, and on a dead flat road, cruise on driving along, my speed wouldnt change, but my rpms would change about 350rpms. it drove me total nuts. It seemed to cycle about every 20 seconds or so. All it was was my timing was off. I cant recall for sure, but i think it was the timing was too retarted.