What's the brand and part number for the new sensor? I'll do some checking to verify that it's right.
I'm with you ... this is baffling.
This is starting to look very much like an electrical problem. So, you may need to have or borrow a multimeter.
One possibility you can check fairly quickly ... look at the back of the gauge (yes, easier said than done) and see how the gauge is connected. If there is one or two wires connected directly to the back of the gauge, I may know the problem.
Most people don't know that older OEM gauges operate on 5VDC. On the instrument cluster printed circuit board is a voltage regulator that reduces the current to the gauges from 12VDC to 5VDC. I've seen cases where people have wired a 12VDC supply directly to the gauge, bypassing the instrument voltage regulator.
Because you've not indicated any erroneous reading on your oil pressure and fuel gauges, I do not suspect the instrument voltage regulator.
