Yamaha outboard overheating.

[Emmalina]

I have a yam 8enduro it was working fine till one day the water stopped circulating.
I have 3 of these engines and apart from impellers dying waterways clogging and carb dirt I have never had a problem like this ! I have rebuilt the water pump and flushed everything even had the head off to make sure the gasket hadn't blown. It has cooling water for about 3 mins then turns to steam so i shut it down ... Any ideas/
Pump rebuild was a new cartridge upper and lower impeller and gasket. But when i try it with a drill doesn't seem to be enough flow...