79 century boat 350 motor

by howard
(ludington,Mi,)

I have a 1979 century boat with a 350 engine both engine and lower unit has been overhauled. My problem is at idel when trolling my boat will over heat unexpectedly this happens once or twice while fishing. some days it may not do it at all. I have replaced the water pump and themostat in it and still having same problem