To me if you say the radiator stays cool nearly ice cold when the engine is running it has to be blocked. take off top and bottom hoses stick a hose in the top and make sure the water passes through it ok. If not that is your problem. If you cant unblock it yourself take it to a radiator specialist they will unblock it for you and check it is working as it should be. If you are running quite big power I would advise you get a new high flow /capacity aluminium radiator. Keeping Cosworth engines cool is always a problem and the more modified and powerful you make it the more you need to do to upgrade the cooling system.