over heating

I have a thermo electron concuerer crusader 220hp 302 .
The engine seams to be over heating intermitently for the last couple of years . Last year I thought I had it fixed . When I went to Green Lake it ran fine the first day but over heated the second . Then it came backdown and ran fine . I checked all the hoses for crud but didnt find anything but little pieces of rust scale . I blew through all the holes to be sure they were coming out where they were supposed to .I have replaced the impeler and the thermostat . I have not replaced the circulating pump . I have not had an impeler loose an ear since I have owned it for 13 years . I dont think there is any blockage

today I ran it in the driveway with a water hose going into a bucket and sucking the water out with the intake hose that was disconnected from the bottom of the boat.
it stayed around 145 but occasionaly rose to 165 . Should it stay lower . Any help would be helpful. Does the impeller have to have a prime before it will suck water ?

Thank you
Pat

Impeller seems to be working ok . It pumps about 4 gals. in 15 seconds . I opened the petcock on the side and reved up and water sprayed out . Then I put my finger over the petcock and felt pressure, This leads me to believe that the circulating pump is good . I took out the thermostat and it still over heated . IM LOST !!!!

Thanks, Pat