OK I have a 38 Lightning 500EFI's New raw water pumps, new impellers (old ones were intact) has been pulling water fine to the strainers....... Will pull water great when started, idle and while running. After running fairly hard for 5-10 minutes and come off plane it will not pull water or pull very little and my engine heats up. I can get it to pull water again by shifting to neutral or turning motor off and restarting. once I do that it pulls water just fine. Help... Not sure where to start looking. I have checked the hose that goes through the transom assembly and it seems to be in good shape. Rubber is not old or cracking. Look forward to your insight. Oh and no water leaking in. Bilge dry.

You're intake hard hose feeding the pump may be delaminated and sucking itself closed.
Edit. Just one or both engines

Just one engine. The hose with the wire in it?

I know your on top of it but go over what was done. Marc parts used? Complete impeller change (housing, plate, impeller)? Switch entire units from engine to engine, see if issue follows.

Yes, I just came across this at work on a suction hose, the inner liner had come free and choke off after a short time.
Just an idea as the problem is unusual to die off and then be fine again,unless there's a bunch of crud plugging up the cooler that falls away when flow stops.

Belt slip?

I will check belt slip. No impeller parts plugging system. I changed to new sea water pump, impeller, gaskets etc after it happened the first time. Still doing same thing. I will check the belt tightness tonight. What size ID hose comes off the transom assembly to the raw seawater pump?

1-1/4"

what is water pressure???? I have just been through this and thousands later it was my emi risers clearance too tight. I have put on glm aluminum exhaust and solved the problemos!!!!

btw , my water pressure was high...