After cruising at about 2000 RPM for an hour or so, we took our boat up a river at about 800 RPM. after half an hour the temperature gage showed 240 degrees. The next day we took a half hour cruise on a river at 800 RPM and the gage went to 240 again. theboat is new to us and we don't know what to expect as far as low speed cruising. Is overheading at slow speed a problem with these motors? what can we do to stop the overheating?