Vehicle has new water pump, thermostat, fan clutch. The front of the radiator along with the A/C condenser was cleaned to be sure good air flow through each. Vehicle temp gauge will jump from 210 to 240 in seconds & then back to 210 again in just a few seconds. After driving for a few miles it will then spike to 240 & stay there for the rest of the trip. With heater on high it don't seem to make much difference & sometime just sitting at idle is will drop down to the 210. Why is the temp jumping up & down so much? Someone said it was a sensor maybe near the thermostat? Vehicle never had overheating problems until a garage changed the water pump & one week later the overheating began. Also the radiator cap has been replaced. Not sure what to check next, but this is getting old & its not worth the money that is getting put into this thing.
The engine coolant temperature can't change that fast so it's doubtful you have an overheating problem. It's more likely the wire going to the coolant temperature sensor is rubbed through and grounding out against something metal. You can verify that by unplugging the sensor. Try to do that when it's pegged at 240 degrees. If the gauge stays there when the sensor is unplugged, the wire has to be grounded.
On an '02 model there should be two coolant temperature sensors. One will have two wires. That's for the Engine Computer. You want the one with a single wire for the gauge. Both of them are usually near the thermostat housing at the end of the upper radiator hose.