Car overheating + new thermostat

My Renault 25 has been having cooling problems for several weeks now, yet i cannot figue out the problem.

It has a new thermostat and water pump. Without thermostat, I cannot install a gasket so it simply leaks everywhere but when I refill it while it is idling: radiator, upper radiator hose and lower radiator hose heat up. It seems that the car does not overheat without thermostat (yet, i have to add cold water while it is idling).

With the new thermostat installed, it overheats again. However, the radiator and upper radiator hose heat up while the lower radiator hose does not.. The sides of the radiator also fully heat up, except everything below the lower radiator hose.

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• Without thermostat, seems to operate normally with a heated lower radiator hose;
• With new (and old) thermostat, heating problems and cold lower radiator hose;
• New water pump

What could be the problem?