I used to have the same problem as yours for almost a year . But i solve that problem 3 months ago , all i did was jack front engine up as high as i can then remove radiator cap , start the car and turn the heat all the way . watch for coolant , it keep getting lower and i keep fill it up untill coolant start going back out then i put the radiator back on and done. For the temp gauge bounces up and down i think either water temperature sender getting bad or some thing wrong with the gauge , you can test water temperature sender by dip in hot water . or replace it ( my water temperture gauge act like that ) , i replaced it and it when back to normal .