My 150 overheated once, when the radiator cap came loose and the coolant boiled. It needs pressure to raise the boiling point.
I'm just about out of ideas to help you. The only other weird thing that a Honda scooter ever did that caused it to fail was when a piece of gasket from the gasket that seals the crankcase halfs came loose and floated down to the bottom of the case, covering the oil intake on the floor of the cases, behind the oil filter. The result was oil starvation and a camshaft bearing failure that brought everything to a halt. It was about the oddest thing I ever saw happen in an engine, but another guy on the Yahoo CH250 group had exactly the same experience.
You really should join that group and see what the folks there can suggest.