When the thermostat opens, the water pump begins to draw from the lower radiator hose, which also means that it's drawing through the radiator and the upper radiator hose. If you have a constriction when the engine is running, the usual cause is that a lower radiator hose is collapsing under suction. This is usually apparent at higher engine speeds. In fact, it's usually a puzzle, because the engine overheats at speed, but the hose returns to normal and temps go down when you pull over. In your case, it sounds like the upper radiator hose has given it up. I would change both hoses. I'd also pull the thermostat to check, since it's right there. May as well replace that, too.
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