Cars engine temp keeps fluctuating from normal?

I bought the car 4 weeks ago and have had nothing but trouble with the cooling system. Firstly the car has been sat in a yard for a year before I bought it. I know that was red flags but the car drove perfect on a test drive. First problem was the overflow coolant hose blowing while driving Creating lots of steam! It was brittle to the touch so I replaced that, the resevoir, the cap and the bottom. Thickee coolant hose. I've pressure tested the coolant system and it's holding lovely at 20psi. The coolant resevoir is bubbling and over flowing when at temp and cap is off so I'm unable to do a head gasket test with that special liquid. I have no milky oil, no white smoke and the car doesn't mis-fire. I plugged the OBD reader in and a air leak fault code came up. Also the car is struggling to stay at temperature. When I took the car for a spin with the OBD reader plugged in it was showing 138f coolant temp after driving for half an hour. As soon as I stopped the car I unscrewed the resevoir cap carefully and measured the temp with a thermometer and it was showing 220f. The cars engine temp keeps fluctuating from normal temp and then drops and back up to normal temp again! I'm losing my mind figuring out what it could be. Any help from you guys would help me so much!