Have you tried back-flushing the engine? Could be there are blockages and the water is flow is reduced through the cooling passages and is bypassing the engine. The Universall 5411 in my previous boat had this problem and back-flushing would usually fix the problem.
I have not tried this because another guy did that on this same moter and got water in the mixing elbow, which in turn got the injectors wet. I agree this may be the problem given my previous troubleshoting steps.
FarCry, I have tried leaving the slip and it seems to overheat even more rapidly.
Thank you both for your input.