I have a 2000 Mustang GT with 122,000 miles. At 4,000 RPM's I am getting a bad pinging (detonation). Does not matter if I am at WOT or not. It has been doing this for some time now.
So far I have replaced the computer, fuel pump, MAF Senso, cleaned my K& N Air Intake, have ran some Injector Cleaner through it and I only run Supreme (91 here in AZ & I have tried adding Octane Boost).
Also, when the A/C is running idle sits at 750rpm's, jumps to 1,100 and settles back down to 750.