I have a 1998 mercruiser 5.7 v8 250 hp, I had the exhaust manifold,riser and outdrive impeller changed this spring. The boat ran fine for last month, I was out yesterday running for about 15 mins at 3800 rpm and the engine temp alarm went on, the temp >180's. I immediately shut down the engine and opened the engine hatch and felt the risers, the starboard riser was hot to touch and port was warm. After a while started back up and headed for the dock, keeping rpm's below 3500 and the engine temp at 170. After I got back I felt the risers again and the starboard was hot and the port was warm. I didn't notice anything blocking water intake on the outdrive. Any suggestions??