I hope you told them that it has to be something simple and they don't rip you off by replacing the distributer or something stupid like that. I has to be either a vacuum line or something silly like that causing your problem since it was running fine before the tuneup.
Sorry to hear of your troubles man, and I wish someone could have helped you with this problem before you had to resort to taking it in to get fixed.
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90 Mustang LX 5.0
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