What about a fan shroud? Are you running a shroud?
How far in/out of the shroud is the fan?
a shroud will help direct the air through the radiator to help cooling. You don't want the air to find a way AROUND the radiator, you want it THROUGH IT...
Have you tried running a garden hose through the radiator fins when it's overheating to see if it will cool the engine down. Water can take away much more heat than air and faster. See if that makes any difference.