Buddy of mine had a similar problem, it turned out when he turned on the gauge lights the temp gauge would go up 10 degrees ???
Have you cleaned out the aftercooler and turbo lately ? Clean bottom? I had a pyrometer on my old boat and when it started to grow grass and slime up on the bottom the pyro would go up 10-15 degrees. The old boat had a 41 volvo in it and when the raw water pump was worn/bad impellor the temp would climb. Maybe you have some broken off impellor pieces blocking the raw water flow ??