Its the exhaust elbow sea water passage way getting blocked. The seawater pump pumps, it cant get out of the system into the elbow fast enough, the sea water passes from the seawater side to the fresh water side past the hose clamps and pressureizes the fresh water sytem and it dumps through the over flow. The symptom that the exhaust water flow is diminished confirms this.
Take the elbow off and clean the seawater passages. Be careful, it will break easily. I take the tube bundle out and the elbow off. I assemble the tube bundle and end caps, connect the elbow to it and put a hose pipe
