I too have had some detonation from my 95GT. I have my exhaust done from my headers back (I'm waiting until I do my heads before my headers).
My car use to detonate big time at WOT. Especially in the Texas heat. I replaced my TPS as the voltage was a little high. No change in ping.
I then changed the Coolant sensor which was also reading out of spec. As soon as I changed my coolant sensor my computer informed me that my MAF was shot. I replaced my MAF with a GMS 75mm (same as Pro-M 75mm) and the problem seemed to go away. I had my timing bumped to a full 10 degrees with my chip and when it got hot I still had some detonation. Timing is now at 9 degrees and my car runs fine.
What type and grade of fuel do use? You say you backed your timing down, what did you change it to? When I removed my chip and adjusted just my timing, I found 14 degrees would start to detonate and I limited my car to 13 degrees in the texas heat. I run 93 octane Mobil gas.
My advice to you (after you answer the above questions) is to check your computer for fault codes (If you need help let me know) and then manually check your sensors. Especially the O2 sensors as work has been done in that area.
Hope this helps, post some answers to the above questions.
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