This is probably a stretch but in that swapping and switching senders you didn't somehow use an oil pressure sender for your temperature sender, did you? Sorry but I use mechanical gauges and don't really recall if the two could be swapped. However it sounds like your temp gauge is mimicking your oil pressure gauge to me. Turn your headlights on at idle. If your temp gauge drops, that tells us it is registering a drop in amps. If nothing happens, increase idle just a bit...maybe 500 or so rpm. If your temp gauge goes up just a bit, then it sounds to me like your temp gauge is somehow trying to give you oil pressure info.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end