Is your thermostat in backwards? I'm not trying to be a smart azz, just trying to eliminate that 'doh' moment as I've done too many times.

The fact that its a cyclic issue leads to the only device within a cooling system that cycles....the thermostat.

You could take your thermostat out and try it and see what happens. If it does not surge anymore, you have a bad new thermostat. I don't have a thermostat installed on my boat.

It is easy to check the thermostat. Put it in a pot of water on the stove. Using a thermometer watch at what temp the stat opens and closes at.

My plan was to take the thermostat out and cut the guts out of it. I will be sure to keep everyone posted. i think its going to rain today so if not today then tomorrow for sure. I have the boat clubs memorial run coming up this sunday and its another long run with a boat load of ppl. gotta be in tip top shape for that.

Ok so I have an update. I took the guts out of the thermostat so that the water flow would still be the same at 140 degrees. fired it up and let it run for about 35/ 40 mins. she wouldn't get above 100 degrees. that tells me right there that I had a brand new in the box bad thermostat! the real test will be when I take it out and run it which i'll get to at some point this week and i'll add another update.
thank you all for the advice!

Dyou have the cast iron housing. I had to take a small chisel and knock out the built up rust from mine. I'm looking at stainless housings now.

I drill a small hole 3/32 or less in the thermostat so trapped air can get through the stat

nope I have the stainless housings

What did you find out. Interested here. I was thinking if your thermostat is to high in the housing it may not be getting enough hot water under it to open.?cool fresh water over the top is keeping temperature down.? is the stat in the housing or the intake manifold. Just wondering. I hate boats. LOL but I gotta have one 😄