Yes I have bled the system until I am blue in the face and she fine for a couple of days and then it starts all over again, I'm at a loss

I have recently bought myself a 98 discovery TD5 and discovered it had an overheating problem, on further inspection I found the rad and thermostat were both shot. I have replaced both and now she does not overheat but now when i drive the water in the header tank disappears and I keep thinking...