1999 Merc Grand Marquis Spark knock

'99 Merc Grand Marquis mileage 87K, as the weather warms, I get a distinct "spark knock" or "ping" when accelerating or putting a load on the engine. It gets worse as the weather warms, especially when I use the a/c. During the winter, I hardly ever notice it. I have tuned the engine, new fuel pump, regular maintenance, but can't solve the problem. I burn 89 octane gas, and that helps, but doesn't solve it. A friend said something about getting the car's computer "flashed". ? I'd love to get this fixed without having to burn premium gas all the time. Any thoughts?
Thanks much.