Okay I don't think IMPO that a chip is going to solve this problem. The chip could be your problem altogether. My AOD-equipped '87 with Mass Air conversion doesn't have a stumble or hesitation whatsoever. Foot to the wood = big time go on that car. What is the TPS set to? How about the throttle body; are there any mods to support it? That could be another element. Does the BBK cold air kit include a new MAF sensor? If you're using the stocker it could just be getting "slow". I've seen that before especially ones with over 100K on them. You might wanna reset the computer and idle speed by going over to corral.net and look up their tech article on resetting idle speed. There's no engine codes coming up are there (Check Engine Light)?
Hook up a vacuum gauge to the vacuum tree and get a reading at idle. By using this and knowing what to look for (it's kinda hard to describe) when observing it and playing with the throttle, you can determine if there's a vacuum leak throwing you lean. Check the operation of the MSD box (I have my doubts on those systems, just my opinion) and coil. A fuel pressure check would be the next thing after that. Hope you find the prob!
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch