mid-grade might help in the warm months. I have found "premium" brand fuel to be better in regards to anti-detonation ("pinging"). I would start by checking the wire routing TSB, doing the aforementioned ignition tune up (wires, cap, rotor, plugs), 180* thermostat (if you like), replacing the coolant (if it is old), and doing a seafoam or autoRX combustion chamber clean. Make sure your oil level is good, too, because high engine temps can cause pinging.