Actually my first thought was a flaky thermostat but you mentioned that it's been changed already. It is possible to have one that's bad right out of the box, though.
If both gauges are fluctuating, it probably isn't a bad sensor. Unless, of course, they're both going bad. Again, it's unlikely but not impossible.
Did you do a coolant flush when the thermostat was changed? Maybe there's some crud floating around in the system, randomly blocking the flow until enough pressure builds up to dislodge it.

 

For the cost of a gasket you can remove a suspect stat and boil in a pot of water and observe. But the back up IR thermometer reading would tell what is going on, even if it does not match the exact value of the scanner, it should track the change in values. So if you are reading 175 outside, and then it moves up to 220, etc. It is an interesting conundrum to determine which of the Rover instruments (if any) is actaully working....

What can cause this internally is bad, like head gasket, cracked head, cracked block. So it is worth some diagnosis. Here is blurb from the RAVE, the gauge sensor is negative temp coeffcient, or goes low as it heats up. So a skinned wire could also do it I suppose, that would normally be very rapid, not a slow buildup.


of the sensor is approximately 17 ohms.

As for the ECU engine temp sensor - if you unplug it, the ECU should use a default value from somewhere else, and truck still run. But I don't know if the ECU still sends sensor value to the OBDII. The gauge value should continue to be driven by the single wire sensor. You could unscrew the 2 wire sensor and put it in a coffee cup of hot water and watch the scanner, in key position 2.

Engine Coolant Temperature Sensor (ECT)
13CU/14CU/14CUX
• NTC type sensor
• Resistance range = approx. 9200W at -10°C (-22°F) to 175W at 100°C (212°F). Approx.
300W at 80°C (176°F).
• ECM fault default value = 36°C (96.8°F).
• Located at the top front of the engine, to the right of the alternator and in front of the plenum
chamber.
GEMS
• NTC type sensor
• Output = Approx. 4.7v at -30°C (-22°F) to 0.25v at 130°C (302°F). Approx. 0.7v at 85°C
(185°F)
• ECM fault default value = dependant on value of air temperature sensor
• Located at the top front of the engine, to the right of the alternator and in front of the plenum
chamber.

 

The only way to inspect the water pump impeller is to remove the water pump, if the vains of the impeller have been corroded away it wont push enough coolant.
The impeller is pressed onto the shaft, if it comes loose it will slip on the shaft, yours might be slipping/grabbing/slipping on the shaft.
I dont advise pulling the water pump until you have tried easier/cheaper things first, like a coolant flush.

 

Would a over pressurized cooling system cause this?

 

Nope.

 

The coolant system won't "over pressure" without some help from steam or exhaust gas. Low coolant, blocked radiator, head gasket, cracked block or head. If the radiator hoses are getting rock hard that is a bad sign. I guess you could "flow test" the water pump, remove thermostat, open upper hose connection. Pour water in, at 1000 rpm the water pump is supposed to move 2.64 gallons a minute. Messy.

The water pumps I have taken off of a D1 and a D2 look like this, the impeller seems to be stainless. Plenty of calcium can grow on there, when they start to wobble and leak around the bearing, they are eating aluminum inside. Probaly the calcium buildup keeps a lot more from just spinning. I wuld suppose a "snake camera" could look down in there (Honey, I need to stop by Harbor Freight for just a minute), you could also stick something down the fitting to "feel" the vanes as you rotate it by hand and count six of them.

A mechanic that installs the thermostat backwards could also be a problem, the "spring" end of it goes in side the hole in the engine. Keep sharp eye on the temps, the scanner is the more trusted instrument.