5th gear pinging

[Treachery]

I have a '93 GT with a Vortech SQ-Trim s/c and all of the usual bolt on's one would expect.

Traveling in 5th gear on the freeway, i hearing what i believe is detonation at WOT. What's strange is during hard acceleration from a dead stop, there is no pinging.

The pinging seems to happen when cruising at around 2500-3000rpm. I then hit the gas pedal and i occurs at around 3500-4000rpm. Another strange note is during normal driving, my gas pressure gauge reading drops. Should it rise when the gas pedal is pushed?

I've done the following: clean throttle body; check TPS (0.95- as instructed by the vendor that burned my Autologic chip); clean the MAS; fuel pressure - 49lbs with vacuum on (about 55lbs vacuum off); changed fuel filter; checked all hoses - appears ok.

Existing Fuel components: 190lph fuel pump; T-Rex Vortech pump; 30lbs injectors; 6:1 FMU.

Any help is appreciated.