water restriction 98 Ficht

[Mac3]

I have a 98 Johnson 150 ficht, the other week I removed the water pressure gauge line to do some electrical work under the dash, later I started the engine on the driveway as it has not run for a month (winter down here don't you know) and the water pressure gauge did not move, I took this to the hose needing a prime after its removal. Anyway, hit the water this weekend and up to 4000 rpm and noticed only showing about 20 psi on the gauge, then an overheat alarm. After shutdown the water pressure showed 10-15 psi for quite a long time so obviously a blockage somewhere in the middle of the engine. The pee hole is streaming as usual, the thermostats opening ok. With very gradual accelleration the water pressure builds up to just below normal but the not draining bit must be telling me something, I have oem parts and service manual, can any expert give me a clue where to start or should I just remove all water lines and go step by step.
cheers
David