What'* the timing set at? You might want to try dropping it a couple of degrees just to see if if has an effect.
What temp t-stat are you running?
Are you running electric fans, or the stock pulley driven one? If pulley driven, is there a viscous clutch on the fan? If so, have you tried running with a solid fan mount?
Has the rad ever been replaced? Have you ever put bars leak or anything like that in it?