From our experience, stock valve springs and lifter pedestals are good to better than 6500 RPM...so I doubt that is the source of your miss. Assuming you don't have any obvious secondary ignition problems (burned/broken wires, bad plugs, etc.) Since Marcello mentioned that you are running a distributor: I would take a look at the ignition module as they can break-down over time and become tempermental.
Which ignition box are you running? TFI (small module on side of distributor) or Big Old Ford Box? (just curious)...
either way, if you have a spare module, change it out and see what happens...it appears as if heat is a contributing factor and those definitely are sensitive to temperature.
Good Luck.
Dave F.