performance of car continuing to fade away

[Mr 5 0]

You neglected to give the year, mileage and mods on your Mustang, but generally speaking,
clogged cats could very well be the culprit - or - you may have a Mass Air Sensor going bad on you, although that would (should) trigger the dreaded 'Check Engine' light.
Try some Ford injector cleaner (only available at Ford dealers - about $10. - good stuff). If that results in any improvement, you know it's quite likely a case of clogged or otherwise failing fuel injectors.

[This message has been edited by Mr 5 0 (edited 08-21-2001).]