Install a mechanical temp gauge and put a piece of tape over the factory gauge.
I hate electrical temp gauges. Actually I hate nearly all electrical gauges for critical system monitoring. Their only advantage, IMO, is that they are easier to install.

 

See sensor on left. Remove single wire that goes to gauge. With wire brush or scrubby pad clean sensor top and center stud. Gunk there makes a second path to ground, and makes readings higher. Skinned wire will do same. Ultra Gauge or a scanner read the OBDII engine coolant temp sensor, which is the one on right with square top (two wires).

Also, if the thermostat was installed backwards, not good. Spring goes inside block. Small 1/8 inch hole or jiggle device goes at 12:00 to prevent steam pocket buildup. Spring for a gasket and check. Better yet, a new $10 180F stat. Don't buy the fail safe models. Stats can be heated in a pot of water to confirm temps.

Don't trust factory gauge. Designed to make you calm while you roast the Rover, making possible jet ski payments by Robin Bastid at the shop.

 

If you are using your a/c during these overheats make sure you condenser fans are kicking on with the a/c. New expansion tank cap couldn't hurt. test your coolant for exaust contamination or signs of oil in coolant, (could mean HG!! lets hope not)

 

I'm running my Innova on Live Data while driving and it's telling me the truck isn't overheating so it must be the wiring which I'll go check now as the sensor is new.

In regards to the expansion tank, I bought a new one and new cap but I can't get a good seal with the cap. It never screws on tight. Is this normal?

 

No it isn't normal! If the cap isn't sealing correctly you are asking for an overheat. For each pound of pressure it raises the boiling point about 3 degrees. So with a 15lbs cap it would add about 45 degrees to the boiling point, 212+45=252. Without pressure in the cooling system you are dangerously close to meltdown even in mild temps, you are asking for trouble if it's 90 outside. Who did you get the reservoir and cap from? I would invest in a new cap ASAP and hope it seals on the reservoir. If not you will want a new reservoir that the cap seals tight on.

 

Higgs, Thanks a lot. It looks like I ordered a reservoir tank and cap from different suppliers and so the threading is off. I just ordered a new set from Atlantic British and hopefully this will help it out