well ford guages are mediocre at best. any mech will tell you that. any factory guage is subject to hilarious laughter. But on a more concerned note, I would say, over time, the radiator and heater core and passages as well as water pump and the temp sender all are subject to age and deterioration, so if you have a build up of corrosion in one place, you will have it in another, as well as reduced flow, giving you some pretty crazy readings. Short of replacing entire cooling system, the age old motto applies, if it aint broke, don't fix it. Do the normal maintenece stuff, as long as motor is keeping cool, and transmission fluid is getting proper cooling, and the heater is getting optimal, I would get a pressure test of the system, replace the radiator cap, make sure the bleed pin hole in the radiator neck is clear, along with the overflow tube, and just live with the flaky readings. I am sure it needs new cooling components, most vehicles do at some point, and when it needs radiator and fan clutch replacement, and heater core, and water pump, then fix it. you could spend alot of money chasing this, and it not be life threatning. I use synthetics and the high mileage dexcool (orange) coolant. I have very small flutter in my guage, but it has been for 90,000 miles. And everything checks out to be due to deterioration of cooling system in mine. I suspect once I change cooling components, all will be well with mine. I am sure this is true for most vehicles. PS. mine has heavy duty cooling and never has reached over the O in the NORMAL band on the guage.