Sounds like the classic case of pinging. What is your timing set at? You can do one of two things to rectifiy the situation:
1) Reduce your timing back to stock (10* BTDC); or
2) Use a higher grade gasoline (92, 93, 0or 94 octane).
Hope this helps,
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'
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