Vauxhall Omega Radiator Problems

Hi my car over heats only when i go above 3k revs. idle speed every thing works as it should do. ive checked all the hoses emptyed the tank. i cant seem to find out what the problem is. changed the thomostat to. the water pumps out ...

Can anyone explain what the coolant heating do for the car as mine has started to run all the time and running my battery flat. Have managed to pull the fuse out which has stopped the fault but I dont know what this could do to the car if i...