The pick up is clean, new thermostat put in and the old one tested fine. I don't have a way to verify the temp, other than the fact that it runs at 160 until idle and then heats up, get going again and she cools right back to 160.
Running with the engine cover off I've noticed this. When I slow down to idle and it starts to overheat some of the coolant gets pushed back out of the engine thru the overflow tube and into the overflow reservoir. I talked to a marine tech about that and he thinks it might be the head gasket causing gases to build up inside which would push the coolant out. Does that sound reasonable?
I'm still stumped and willing to hear some more ideas.
Thanks