1986 300E overheating

[04 Diesel]

The hose that comes from the overflow tank to the tank behind the passenger quarter panel broke, so I replaced the hose. Started the car and it started to overheat again, and there was air escaping from the nipple on top of the radiator. So I replaced the thermostat and all was good, I let the car idel for a good 20 minutes and took it for a drive. This a friends car, so she drove it home and all was good. She called me today and said the car got hot again and air is coming from the nipple on top of the radiator again. What is going on with this car?