My 98 Buick century has temp problems?

My buick temperature gauge flucuates and rarely never stabilizes. It gets hot but never to the red and every time Its idling the needle moves. I had my lower intake manifold gasket changed and put different fluid in it. Could that be the problem? Or is it a thermostat or radiator. Also my low coolant light comes on and then goes off but my coolant is filled all the way. please help