[C320-list] Overheating Problem

[Rick Onyshko]

I have a 3 year old Yanmar 3YGM and had an overheat alarm go off on my first trip after the winter.  I replaced the impeller, no missing vanes.  It seemed to run at normal temp ~ 150 for over an hour.  Today I took it out and had it idling while I dropped sails and put fenders out, and it did not overheat but the temp went up to about 170.  Returned to 150 when I started moving forward and the RPM went up.
 
 Not sure if anyone has any ideas or suggestions, it has typically run right at 150 pretty consistently.  
 
 Thanks
 
 Rick