"they all do that sir".
Quite all people around me have this problem.
Plastic or not plastic, due to vibrations, rigid hose and twisting between motor and radiator. It is a recurrent problem on Morgan.
The concern with "plastic radiator": they cannot be refurbished; aluminiun radiators are quite impossible to repair but stronger. Only former copper/brass radiator can be easily repaired.
You have to replace it by an aluminiun one. I don't know if traditionnal copper/brass radiator could be found for a Roadster.
