if i missed this at the start, please forgive me. [just an old man wondering through the cobwebs of my mind........ good title for a movie or a song, eh ? biggrin]
did you try a different brand gauge and it's corresponding sending unit ? [i know you tried a different sending unit with your existing gauge]
did you try a known good mechanical gauge ? [if there is a place to install it.]
how about using an infrared heat gun to check the top and bottom of the radiator, as well as the heater hoses, to get some idea of how hot the engine is actually getting ?
beer