I remember in a Mustang mag. that the tech mentioned that there was a particular computer ID # that was junk. It was in the 90-93 fox bodies and that it wouldn't even accept the performance module without freaking out.
Maybe you're thinking of the '93 Cobra EEC which had more conservative tune than the regular '87-93 5.0 HO, among other things. Its generally alot less convenient to use due to its programming. If thats not it, I ca safely say I've never heard of this supremely rotten computer you speak of.