Sometimes ... it's more than one duck.
■ If you have a pocket of air trapped behind the thermostat it won't usually open, thus preventing the water from flowing correctly. If you have a pocket of air at the water pump it will cavitate, and water won't flow. If you have sludge or mineral deposits (you DO use distilled water, right?) they'll build up and act like restrictive insulation and you can overheat, even if water flows. If you have an old flimsy lower hose it can sometimes collapse, preventing the water from flowing.
So, as an old fart I usually drill a couple 3/16 holes in the thermostat before I install it. This lets the air escape right on thru, and it also allow a little water past so the thermostat acclimates immediately.
You know how the temp gauge bounces around? That's pockets of air mixed in the water. Mine used to do that. With the new and holey thermostat it just climbs to 1/3, and stays there. My advice? Trust nothing, check everything.
Pull the thermostat and put it in a pot on the stove, with a candy thermometer. Make sure it opens at the right temperature. While you're doing this put the housing back on, add some coolant system flush, and see how much crapola comes out. While you're doing that, check the fan and the fan clutch. Does it actually work and spin the fan, is there a lot of air rushing thru when you first start it and again when it gets hot. When you changed the temp sensors on the radiator did you check to see if they work, does that front fan really work, and does it work like a two speed fan? Did you make sure there's a clear air path, from the front of the car, thru the AC and water radiators, and out? Did you check the hoses, make sure they are fit functional not pinched or dented and they're routed correctly, if there is a bypass hose make sure it's not clogged with goo, and make sure the overflow tank is half full *all the time* until this is working properly.