Temp Issues with 3.0L

[Harritwo]

We were out fishing the Redfish Tournament last weekend when my temp guage went to 100 deg F. I checked and ensured that i had sufficient waterflow and put on the trailer. After getting home, I pulled the T-Stat and changed it. When we fired off, the guage was still showng 100 deg F. A check with an IR thermoter showed actualengine temp at the T-stat housing was 142. I suspected a faulty sending unit. I pulled the sending unit and ohmed it out, When heat is applied, there was no change in resistnce so i replaced the sending unit. When we fired off, the guage was now reading 185 deg F. A quick check with the IR thermoter showed an engine temp of 142 deg F. I chaged the sending unit again, this time the guage is reading 145. ABout 5 deg differene between the IR thermoter and the gauge.

Has anyone run across this and is this a normal variance or did I receive a faulty sending unit on my first purchase?