water tmp dives at high speeds

Hope someone else has heard of this problem. I have a 1996 Chaparral 24 ft. Signature, 5.7 2 brl. raw water cooled. Boat has always run great, never a breakdown. Strange problem started in the middle of last summer. Boat will idle or run at low speed all day around 155 to 160 degrees. As soon as jump on plane temp goes up about 10 degrees then quickly dives to lowest point of gage. Within 3 or 4 seconds of coming off of plane gage goes back to about 160. Never shows overheating condition. Did not trust the gage so I put a mechanical gage in where temp. alarm sensor goes. It did the same thing. Last spring I put a new outdrive water pump in, new manifolds and risers, and I have tried 2 new stats with same results. It almost seems like Iam getting an air pocket. But if it was I believe it would show a higher temp when coming off of plane. Local techs. have not seen this before. Any suggestions would be greatly appreciated. Thanks

Re: water tmp dives at high speeds

Pull your thermostat housing off and up inside there is a bypass hole. You can't see it, but it's in there. If it gets plugged up by rust or debris from an old impeller, you will get that exact problem. Finding the hole by feel and cleaning it out is the fun part.