Q: Engine cooling - 1996 Ford Mustang

I noticed that the car had been losing coolant. It has not been showing signs of leaks. One day I filled the reservoir after running it and let it sit overnight. Next morning it had rained so I couldn't spot leaks but the coolant had lowered. Then I filled it up again drove the car around the next day and let it sit overnight this time no coolant had leaked. The car has cooling issues. when it is driving down the road it is fine but when I let the car sit in neutral or first the temp gauge raises. I know that the fan is turning on because I let it warm up and the fan kicks on. Later this night the coolant was filled and I drove the car around I tried to turn on the heat at first no air would blow but then after a bit air started to blow but it was not hot. It was cold. Hour later coolant had dropped. The oil in the car was just changed it has no milkyness. All I know is the car is losing antifreeze when I try to use the heat I guess and it runs hot when it's at an idle. Thanks

My car has 140000 miles.
My car has a manual transmission.