Late Response to engine problem.
Sorry folk, but we've been cruising the Gulf of Alaska while waiting for better conditions on the Gulf Coast of the lower 48.... Had a Whale of a time!!
The slow overheating problem sounds to me like an impeller deficit on your water pump. Basically, you are getting some cooling, but the volume isn't up to spec. and eventually the engine overheats. Pull the pump and take a look. Don't need a mechanic, just a wrench.