1998 Ford Mondeo lack of power unable to start

OK, i'm having probs trying to work this out, When you first turn the key in the ignition the dashboard lights up really bright, you turn the key to start the engine the lights on the dashboard Dim, the starter motor clicks but it don't start the engine, We have managed to bump start it really easy, but whilst running it seems to be draining power as it needs to be constantly kept at a certain rev or it stalls, We know its not the Battery or the Altinator as both have been checked and working fine