You say, when all the others radiators are off the problem radiator is ok.
I suppose that could rule out a valve problem then.
That must bring us to the pump and I can understand with only one radiator open the pump will be 'man enough' to deal with it.
But what happens when you have 'two' then 'three' then 'four' radiators open, at what point does it begin to fail.
I know pumps are available to cater for heads of '5 metres' and '6 metres' so it could be your pump is not 'man enough' to deal with all the radiators.
Mind you I don't know how many radiators you have or the height that the pump has to push the water up.
Perhaps placing the pump on top speed will make a differance.
You havn't said where the new radiator is fitted, it's not in the attic by chance.
I could understand if the was resistance due to too small diam. pipe but if the pipe at the radiator is hot it does not make sense(well not to me)
I'm still thinking along the lines of a faulty valve, with other radiators closed all the pumps efforts are directed to the one radiator so that little extra pressure could be the difference.
I would still verify if valve is fitted correctly