Re: 4.3 v-6 yamaha drive
Have you verified that you have good flow the the eng.
If you do then there maybe a small leak in the system.
I'm not real familiar with the Yamaha drives but what you describe sound like the impeller is not pushing enough at low rpm(mercs and omcs will do the same thing with an impeller that is on its way out)so I would check the system for leaks and verify the pump was installed correct.