You should try running it without the custom chip first. I removed the Hypercrap from mine and found oodles of power! The mass air conversion helped out, too. Are you certain that the computer is getting constant battery power? This alone could be the problem of why it keeps searching for an idle and not kicking out any codes. On the other hand, if the chip is interfering with signals the computer is looking for and it receives the correct ones, it won't give codes, either. I'd try resetting the idle first, WITHOUT the chip, and see how far you get. Good luck.
p.s. If you still think it's the computer, try using one out of a friend's Mustang.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch