Hi there,
Try checking this: fill the radiator completely full, then run the engine until it is completely warmed up and check for bubbles coming out of the water. "goose" the engine a few times while checking - once the coolant system is completely full and the air is purged from the heater, etc. there should be absolutely no bubbles emanating from the system. If there is you have a blown headgasket or a cracked block or head. (Usually a blown headgasket, 95% of the time). The combustion gasses will "blow" the coolant right out of the radiator while you are going down the highway, and the engine will overheat, the heater will stop working because there's no coolant flowing through it, etc.
Good luck with it!

RGW
'82 300D/4.3L V6
Etc...