Goofy engine Temp issues

[93liftedXLT]

I've got a question about engine temp. once my engine heats up my temp gauge slowly bounces between 130 degrees and 240 degrees. It stays at the L for about a minute and goes back to normal. Anyone else have this issue? No overheating. Maybe thermostat is going bad?
4.0 manual.

 

[1993Saturn]

I would suspect that due to the famous Explorer air bubble. The coolant doesn't always conveniently fill up all the nooks and crannys of the system. There can be a big bubble in the heater core which can sometimes cause a thumping there. There can also be one over the thermostat where the temperature sensor sits. When the coolant fills the void, the temp reads normal. When the bubble appears, the thermostat is now sitting in hot air and the reading skyrockets. Ford long ago had secretly offered a solution with a bypass hose from the top of the manifold meeting up with the pump return hose to the lower radiator. No longer available, hence we must suffer with this particular quirk. I had installed a Prestone heater hose "T" in the upper heater hose. It has a lawn hose threaded cap on it used for flushing. It also worked well with filling up that void when refilling the system. I badly discovered you better replace it every year and a half or so or it will blow apart in about 2 years.