Well there's your problem!
(kidding!)
Seriously, though, where's your temp sensor? In the head or off the thermostat housing?
In this pic you can see mine in the head. It's on the driver's side right at the #2 piston. It's the brass plug looking thing with a wire coming out of it.
If it's there it'll read high at idle because the exhaust manifold butts up to it. When you're driving air flows around it cooling it down.
I've replaced my radiator (3 core), thermostat, and water pump and mine still creeps up because the sensor is about 1/4" from the header flange. I know it's a false reading.
Yours could also be a bad radiator. You can try flushing it but I doubt you'll get the gunk out.
