Temperature gauge fluctuations

Nothing. There is a bleed hole in the thermostat that likely became blocked with debris that got knocked loose. That bleed hole allows the coolant that is hot "over there" by the sending unit for the gauge to migrate "over here" to the thermostat. When the bleed hole is blocked the hot coolant reaches the sending unit like normal, but it takes a while before the heat gets to the thermostat to cause it to open. Often when the thermostat does finally open, cold coolant comes rushing in from the radiator and the thermostat closes again. That cycle can repeat a few times before the system stabilizes. It won't hurt anything but it can be annoying.

My '88 Grand Caravan actually started doing this many years after the thermostat was replaced. Drilling a 1/16" bleed hole solved that.

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It could clear up if the blockage dissolves or falls out of the bleed hole. It's also possible the mechanic got rather aggressive with the flush machine and tried to do a really conscientious job. That extra force that is supposed to break loose any caked-on sediment could have wedged the debris into the bleed hole even tighter.

On some engines you can see the thermostat very easily after you remove the upper radiator hose from the thermostat housing on the engine. The older Chrysler 4 cylinder engines from the 1980s and early 90s were like that. Those thermostats sat on their side so debris would fall off and not block it open. They didn't need a bleed hole because the sending unit lived right next to the thermostat. That means when the thermostat opens, the sending unit, (and dash gauge), were just getting to the normal reading at the same time.

Another thing you should check is to see if the reservoir is about half full. If there was an air pocket in the cooling system, which happens kind of often, the coolant would have been drawn in from the reservoir when the engine cooled down. If there is still more air in there, that can prevent the thermostat from opening on time. Thermostats open in response to hot liquid, not hot air. It could take some bouncing over bumpy roads before a little hot coolant splashes onto the thermostat. Once it starts to open, higher engine / water pump speeds will convince the hot coolant to flow up to the thermostat to keep it opening. By the time that happens the sending unit may have already been hot for a minute.

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