Mercury 9.8 110 thunderbolt cooling problem

[Oldguy44]

I’m having problems getting the cooling system to have a good steady stream of water out the telltale. I have replaced the impeller replaced both gaskets on the exhaust plate and cleaned out everything on the head. Air and water (with garden hose) flow good. I pulled the powerhead and cleaned good and replaced gasket. It will have a good constant stream at a low idle but when you give it gas it starts spitting air and water.does anyone know what the problem could be. Thanks for any help