I give up

have you gave it a coolant flush?

is your lower or upper hose cooler than the other?

you may have a plugged up radiator.

when I suspect a plugged radiator or poor flow I will unhook the lower hose and I will take the radiator cap off and stick a garden hose in it and let it flow and see how well if at all the flow is on the lower end.

also, its a good idea to go to your parts store and pick up a flush T and install it in a heater hose. (also aids in bleeding air from the system).

here is how I bleed out the air in my cooling system..... I will start the engine with the radiator cap off.... Then I will let it warm up and I will keep adding coolant as needed..... then after it has warmed up a bit I will lean over and hold the throttle open at around 2,000 rpm (this will get the water pump turning fast and suck the coolant level down......at which point I will add more coolant (still holding at around 1500-2000 rpm) and top it off.

repeat if needed when stat opens.

also, are you getting any white smoke? you may have a head gasket leak if so.

OR are you getting any bubbling in your coolant? and or a fuel smell?

I had a vehicle once that had a headgasket fail that allow the hot exhaust gas's to jump over to my coolant port and it made my coolant smell rich and over heated my engine.

just guess's