Description reads like a faulty gauge to me. I might have considered a loose contact nut behind this gauge but... "sometimes points below the dash". At off the needle should go to the same home position every time. Sometimes it stops at the C and other times well below the C ? Bad gauge.
The same ohms range and same tester applies to all these gauges and senders. 80 ohms is no needle movement. 73 ohms is visible movement to a low position ( like the first has mark ). 23 ohms is near the center of the range/screen. 10 ohms is the high end of the range.
Shorting the sender wire to ground is zero ohms. That can generate heat and movement that normal within 80 to 10 range cannot.
