Engine stumbles and misses under normal acceleration

Plugs are cheap and easy to replace so I suggest doing that in the meantime. I had the same issues with my car once and I didnt know it but one of my plugs got cracked when being installed. They looked good and I couldnt see the crack very well and felt real silly for waiting so long. Just a thought.
 
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My 89 GT did the same thing for a long time, I replaced everything, IAC, TPS, Plugs, Wires, Etc. I finally took my EGR off and cleaned it with carb cleaner and and sprayed the crap out of the ports on the spacer where the EGR goes. I then started the car for a brief moment gave the throttle a quick snap to blow the crap out of the intake runner, replace the EGR gasket, it is NOT reusable, my stang has not misfired or stumble since then, I hope this fix works for you too.