Is this immediate right after you start the truck? Once warmed up if you shut it down and restart in 10 - 15 minutes do you have the same issue?
Do you have access to a IR temp sensor/gun with a 15/1 or 30/1 spot to distance ratio? The better ratio will give you a smaller area for a measurement. 30/1 is a 1" spot at 30". If so you could shoot the temp in the location of the sensor and get an idea of the temp that the sensor is seeing.
Has the water pump been replaced with a newer style? I seem to remember that there was some issue with the old style pump with the short stem t stat and a potential over heat issue. It has been to long ago and I do not remember the exact concern with the older pump.
I would lean toward an electrical issue until you said that the Isspro did the same thing.
One other thought is do you have the correct temp sender? There are two options for our trucks. One with a red band, and one with a green band. I just went through a similar issue last year on my 95 F350. When I would first start it the temp would be fine but after 15-20 minutes it was reading 80% of full scale. My IR temp gun was only showing about 185F. My sensor had failed and I picked up a new one and it did the same thing. I thought I should have checked the dash gauge. But I did more digging and found that the replacement sensor was not the correct one.
"Ford used 2 different styles of temperature sensors on the 7.3L engines that look the same but function differently. If you touch your sensor wire to the chassis and your temp gauge reads full scale, you will need the 6-016 sensor. If it does nothing, you will probably need the 6-015 sensor.
There is also one difference in appearance that may be hard to distinguish on the sensor once is has been installed on the engine for years. There is a colored ring in the sensor around the single electrical pin. One is red and the other sensor is green. If you have a green colored ring, you need the 6-016, red is the 6-015 sensor."
Link to this info:
https://www.dieselorings.com/1994-1...s-parts/6-016-coolant-temperature-sender.html
