>Since you have changed the resistance quantity (removed the vented loop – reduced pipe length and # of bends) and
>the temp alarm did not sound then you now can assume that you can solve the problem in one of 2 ways.
The test that Jud did removing the vent did not seem very conclusive to me. He needs to run the engine WOT for 45mins or an hour to see that it does not overheat in that case.
The saltwater pump on these is engines is a really simple device. I really doubt that the pump is the issue here. I'm still betting on doing a coolant side flush as the answer.

Paul L