I have a 2000 GT with 35K miles. When I wind out the engine in 3rd or 4th gear to greater than 3500 rpm I can hear a very noticeable pinging sound. The only mod to the car is a K&N air filter.
run some injector cleaner thru it... if that doesnt work then seafoam it...
if your not the original owner check the crank pulley, does it look like it has an extra piece with a pointer on it? if it does then you've got a timing adjuster and its probably cranked up.
I've seen this happen from some odd things before. One, it was one of the coolant sensors acting up, making the engine think it was on fire. Another time, the MAF was filthy. On this Taurus I worked on, it was the fuel pressure regulator.
I'd also make sure you're not getting jerked around with the gas. There was a station near me that just got in trouble for having 87 in all of the tanks, and selling it as premium. This ethanol blend crap doesn't help, either.