2004 Cadillac SRX Sometimes sluggish/pinging

Over the last 20000 miles this has been a growing problem. I use the highest octane gasoline I can get. At low engine speeds, under light load, slight throttle, it pings. The warmer the weather the more it tends to ping. It is getting progressively worse - pinging at higher speeds in cooler weather but still at light throttle. At higher speeds, it will ping loudly after an up-shift and lower engine speed, again with light throttle. Heavy throttle at any speed or gear immediately stops the pinging. Power seems to be sluggish at moderate speeds, 30-40 mph, from a light throttle with or without pinging present. It seems like an inordinate amount of throttle is required and often there is a significant delay in the engine finding power. I had my mechanic look at this a few months ago. Then he found a big vacuum leak and fixed it along with running some sort of cleaner through the engine and fuel system. It seemed to help for a while, but it never did eliminate this behavior. He said that if this doesn't work he'll try wiping the car's computer and completely reprogramming it. Oh, one more thing, this pinging at low speed thing is much more evident and frequent after setting at an idle for a long time.

Thank you very much for any light you can shed on this. I've got to make this car last a while longer.