Temperature gauge fluctuations

I have a Toyota Pickup 1993 3.0L 4WD 270,000KM manual transmission.

I recently flushed the radiator because the coolant was looking a little brown. After I flushed the rad, the temperature gauge fluctuates all over the place, and unpredictably too. Before the flush, the temp gauge went to a certain temp and stuck there. Now, it'll rise when the engine is working harder but then fall all of a sudden, with the engine still working hard. Generally it's rising and falling unpredictably. I topped up the reservoir, and tried to expel all of the air. What is going on? Is there still air in the system? Did flushing it dislodge some sediment that is making the thermostat falter? Is it the water pump?