I bought a 2009 ***** tonk. I really enjoy this bike.

When I am in the big ring and pedal standing up, the front derailer moves just enough to rub the chain.

I have tightened down the left shifter as hard as it can go, but it still moves. I assume it is from the frame flexing, but I am not quite sure. Any suggestions how to prevent this annoyance?

Note: using derailer in homage to Sheldon Brown on April 1st.