1993 Ford Explorer Ford Explorer temperature gauge flucuate

I installed a low mileage Ford Factory rebuild engine recently in my 93 Explorer. It supposedly has less than 10,000 miles on it. Put in a new water pump, thermostat, radiator, coolant, radiator cap and hoses. Fan clutch isn't brand new but not that old and a heavy duty version. The fan is loud under throttle. I burped the radiator by leaving the cap off while bringing it up to temperature while adding coolant. Still wonder if there could be air in there. The temperature guage stays cold and right on the mark of where it should be under heavy throttle. The guage jumps up quickly from time to time at light throttle or driving around town, then rather quickly comes right back down. Heater works, coolant is not down, no steam, no obvious overheat, no muddy oil, coolant looks normal, no boiling or steam bubbles. I replaced the temperature sender with the old one from my old engine. No change. Thinking of buying a new one. The old motor blew a head gasket and may have damaged the catalytic converter, but I am thinking that wouldn't cause such a quick spuratic movement of the temperature gauge, if there is a problem with the converter. Not sure what to do next or what could be wrong. Any suggestions out there?