I bet the water pump isn't pumping enough at idle. DO you have a hi-flow thermostat? They really do make a difference.
Get a fms cobra overdrive pulley for the waterpump. Keep the underdrive crank pulley, and you'll have the same ratio as stock.
If that doesn't do it, try a better pump. I highly recommend the
www.stewartcomponents.com pump. I was using a FMS-C50, and the right bank ran noticably hotter.