It has to do with the warmer temps, lower octane and the fact that they put methanol or ethanol in the gas to reduce emissions. If you haven't done a fuel injection cleaner, that may help. If you don't want to spend the money on a supercharger and water injection, just buy 100 octane at your local 76 station. It is only $3.95 a gallon. That's a long way to a supercharger.
Pinging is from pre-detination. It can be caused by a carbon deposit that actually ignites the fuel prior to the spark. Since higher octane fuel burns slower it slows the combustion front down enough that the piston is heading on the down stroke when the front hits the piston.
I would think that the knock sensor circuit would eliminate the pinging by retarding the timing. If this circuit is malfunctioning or the distributer is so miss-adjusted that it can't compensate then pinging would be uncontrolable.
If the timing circuit checks out then I guess carbon might be at fault.