My 04 grand cherokee 4.o temp gauge will show hot when first starting up. That is right away zero time for engine to warm up. While driving it may or may not return to normal and will sometimes spike to overheat and set off the check gauge alarm. The engine is not overheating and coolant is full. A new thermostat has been installed . Could this be a bad sending unit, gauge or bad connection.

You posted in the XJ section of the forum. The 04 grand is likely quite different to the XJ so you might want to post in the grand cherokee section to get the help you need.

Maybe one if the mods can transfer your thread there.