You should deal with one system at a time. Figure out if it's the EGR and then move on if it isn't.
There's no reason you should be pinging with 91 octane at 10* of timing. You, in fact, should be running 87 octane. Anything more is a waste of money and will foul your plugs eventually. Premium is only called that as a marketing trick. It's not cleaner or better gas than 87. It only takes more compression before it spontaneously detonates and burns slower.
What kind of spark plugs are you running?
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351W 89 Mustang GT Convertible