[rclampitt]

What I should have said is, "the salt water flow through the exhaust would abruptly totally stop". Ocasionally, would start again, without shutting engine down, within a minute or so, before overheating. Usually would have to slow the boat and restart the engine which usually worked. In one case, had to fill the salt water strainer to get a flow. Acts as though air gets in from turbulence, although no reported previous history of this. Will check heat exchanger as suggested. Also intend to conduct pressure test.