Intermittent flow is usually caused when a boat has a wet exhaust/silencer. The water comes from the manifold after the heat exchanger, then down to a wet exhaust / waterlock muffler (has chambers and weirs inside) builds up in there, then when full is pushed out by exhaust pressure, with a spurting motion over and over. There are different types of wet exhausts, but these are probably the most common. I had one in my 20ft NZ Hartley cabin boat, on a marinised MK 3 Zephyr Zodiac 6 motor and it worked well.
Things you might want to know about exhausts
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If a boat has a tell-tail, the water volume will usually vary slightly with motor speed (like an outboard) but is self limiting , as the rubber vanes on the pump impeller will fold back at high revs keeping the flow steady (if they didn't, the pressure would probably blow all the hoses off, or blow a head gasket. High performance tunnel hulls etc with straight exhaust headers, have wet exhausts with a sleeve on the exhaust running to the outlet, (which keeps the outside cool) and the water falls out round the exhaust stream.
Some marine engines like MerCruisers have a riser as part of the manifold, with water fed into the exhaust after it . Lots of different types of wet exhausts, but most work the same way, by water cooling the exhaust externally or internally (which reduces sound) and removes gasses.