temp sensor

Mopar340dave(or anyone),
Using this old email to launch a related request for help.
I just installed a new stroker motor back in my 74 duster a few weeks ago. After struggling with header/exhaust connections and a few oil leaks, everything (almost) is working fine and I now have a few hundred miles on the engine.
The almost...is that my factory temperature gage started to act "wonky" when first starting engine and now has completely quit. By wonky I mean that it initially pegged to full right (off the scale) on start up and then would move a little bit to the left (cooler) range on gage after driving for 10 mins. or so. That persisted for 3-4 trips and then it simply pegged at full left(off the cold end of the range) and stays there. (I've been measuring temps with a gage built into my radiator cap and they have been at around 180-190-deg whenever checked).
Some background:
--All other gages(ammeter, fuel, speedometer, oil/brake lights) work the same as they did before installing the new engine.
--New intake and old intake were both aluminum
--I was initially using my old sensor in the intake but decided to replace it with a new one(Gage remained INOP and stays pegged all the way to the left)
--I measured voltage from the sensor wire to ground and with key turned on it is at 12Volts.(not sure what it should be?)

What other troubleshooting steps can/should I take?
---It's certainly possible that the dash gage has simply gone bad(it looks like the original 1974 unit) but I want to confirm before replacing it.
-THX Scott