[Blas]

You should also verify that you have the thermostat installed in the correct direction, with the small drilled hole(s) in the thermostat flange at the top to allow trapped air to pass. The final coolant level in the expansion tank should be right around where the expansion tanks internal plate is (it has a hole you can feel and see inside) I should automatically adjust itself after a few heating & cooling cycles with the excess coolant being passed to the overflow tank (if you have one) Many of the overflow tanks are not the recycling design, and when they overfill, coolant sprays out of the little breather hole on the top making a mess of the engine compartment. Most overflow tanks have a valve on the bottom to empty them.
Now, regarding the temperature swings; the fans are controlled by a temperature switch located on the radiator on the driver’s side, lower, inside corner. It has two wires (grounds) that trigger a relay behind the dash that activate the fans. To check this switches operation just remove the two wires, connect them, and both fans should operate. (The car can be on or off). If your fans are not turning on or coming on at a higher temperature than in the past, this switch could be the problem.
Another thing; Since you mentioned that you had work done on your car that may have required the fans or the entire fan shroud assembly being removed...The first thing I would do is be sure both fans are operating; and that the fans are both turning in the correct direction. They both should be PULLING air into the engine compartment THRU the radiator. This is a very common problem.
Feel free to contact me if you have a problem with the fans electrics. Send me a phone number offline.