Car overheating

My car suddenly started overheating, the heat in the car at the same time started not working correctly taking longer to turn on and when slowing down or stopping will turn cold. My fans are not turning on as they should either. The motor has been checked and the water pump too, they are fine. Have checked for leaks but have not found any. The car usually has to have water put in every other day sometimes not a lot sometimes up to a gallon no matter how far I have driven. As long as the heat runs and I am driving steadily the car stays at the 3/4 line where it has been since I got the car which the car company told me was normal. My father thinks it might be the ECM/PCM but i'm not sure and want a opinion before paying for that part.