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You may be on to something....after a bit of googling, it seems that copper(actually all metals) tend to be less conductive as temperature rises within the material.
Another thing that may be part of what you are seeing is that the oxide layer that forms on the copper when exposed to high heat and free oxygen can't help conductivity from the tip to the wire even though many copper-oxides are conductive. I suppose a fresh, brushed tip will serve better than an old tip that has been heated to a temperature where an oxide layer has formed on the surface.