2000 Mercury Optimax 135 overheats at low speed
 

I have a 2000 Mercury Optimax 135 on a Boston Whaler Dauntless 18.
Started noticing elevated temps on the gauge during slow running.
Approx less than 2000 rpm.
Running higher than that will bring the temp down no problem.
The best way to reproduce is run WOT to fishing spot, slow to 1500,
continue for a couple of minutes, bam the temp will move up steady till
the audible motor alam sounds and temp gauge is high.
Replaced impeller, may have improved slightly, but did not fix it.
I have read several posts with the same issue with this motor but did
not see any exactly the problem.
Thanks in advance for any ideas.