Engine Temp sending unit

So the temp gauge in my 84 LX 5.0 started reading sporadically then quit entirely. Tested the gauge by grounding the sending unit wire and it works fine. Replaced the sending unit but gauge now reads at the top (hot) end of the dial, even though actual temp is normal. On the off chance I got a bad sending unit, I got another which does the same thing. In 2007 I put a GT40P crate engine in the car, but used the original intake manifold so the sending unit wasn’t changed, and it’s been working fine since then (until it didn’t).
I don’t know what to do next.

It is a single wire sender. The reason I don’t think the wire is grounded is because the gauge starts out cool, then gradually increases as the car warms up. If the wire was grounded it would go immediately to hot when the ignition is turned on. When the gauge is on full hot, an IR reader shows about 185 at the thermostat housing. I’ve read that the ohms reading should be 74 at dead cold to 9.7 at hot, but I’m not sure how to read it.