I have a 1990 Mercruiser 454 Bravo 3. I got it last year. It has been running great all summer. Just this week I had an alarm go off in the dash. I noticed that the temp gauge was much higher than usual. I let it cool off before restarting it and it dropped back down again. It has been doing the same thing since. Seems to run fine at idle and lower speeds but when i open it up the gauge begins to rise after 10 mins or so. Normally it dosent move from the 100-125 degree area but lately its been going up over 150 and 175 so i shut it down and let it cool off before starting it again. Any ideas? Never had any problems before. What should I check first? Im assuming its an obvious thing but I'm new at this. Thanks Guys!!!