Exactly what "cossie" radiator is it ?

OEM, cheap copy, chinese alloy ? quality ? other ?

If the problem is at low speed or even cruising etc....then power you think the engine has, has fuck all to do with it. Because at cruise...it isnt making xx bhp.

Airflow is all important...only cool air in the front and through the rad, never around it. And hot air has to be able to escape. The fact you say the problems started when you changed the radiator....well...has to point at something to do with that radiator install.