Biopace chain rings and front deraileur rub
Keep in mind that there are 5 adjustments on a front derailer:


Low limit
High limit
Cable adjustment -- selects where the derailer stops for intermediate cogs
Height above crank
Angle on seat tube


For subtle chain rub problems sometimes adjusting the angle (very slightly!) is the best approach.

But remember that if the rub is only once a revolution it suggests that the ring, arm, or crank is bent.