Got an 89 Sea Ray 220 Sundancer, Mercruiser 5.7L, ALpha 1 Gen 1 outdrive with 14.5x17 aluminum prop. Engine was turning only 3900 rpm @wot, sooo I went down to 15x15 ss prop (Quicksilver). Rpm's came up to 4700 + rpms. Here's where it gets interesting. I now go up to 14x17 ss prop (Stilleto) to try and bring rpms down a little. The rpms go down to 4500 @ wot. I shut the engine off for about 15-20 min to go swimming. Start the engine up, rpms @ wot are 3900. where did 600 rpms go? I didn't touch a thing- honest! Engine runs fine. Don't think it is the tack in error as I can hear the difference in engine rpms between 3900 and 4500. I noticed that the rpms during the rest of the day on the lake would vary 100-200 rpms without rhym or reason. I'm new at boating and this is my first boat, had it about 3 months. Any ideas from anyone would be greatly appreciated>