On my 67 Mustang GT with a 390, the engine runs smooth at idle and normal driving conditions. During hard accelleration and higher rpm, the power tapers off quickly, and the engine gets very rattley. It sounds like valve train noise. It is not timing, it will just barely ping so it should be right on. The oil pressure is good- ie I have a guage that reads normal, the pressure is not going down. Could it be valve float, or lifters collapsing?? I know this is hard to diagnose like this, but any ideas??