One sure way to find out if it is t-stat related ... is of course to remove the t-stat.
And you could probably rule out any rpm or air movement issue if it does the "overheat then cool down" behavior while just sitting in a driveway idling.
Have you noticed if the heater works when the engine gets hot on start up?
I see what you mean ... strange problem ... sounds like something crazy like the water flowing the wrong direction
