Ignition problem.....

Missing under boost.....had Magnecor wires on,replaced them w/Ford oem's miss went away for about a week..now it's back.Checked plugs -nothing obvious,replaced DIS,car still misses btw 1500-2000 rpm under boost.I know that this is usually plug,wire problem but why would miss return after a week?Any ideas?
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Have you checked for any stored memory codes. Also if you changed plugs recently you might have possibly put a hair-crack in one of the insulators and they (miss, idle hesitation) become worse on damp days.