Could be a fuel problem, could be a weak ignition, could be a problematic aftermarket ignition, could be shot wires etc...
When was the last time you changed your fuel filter?
When was the last time you did the plugs/wires?
Is your ignition stock? If not, remove any ignition boxes or gizmos and see if it works with the stock ignition.
When did the problem start?
Did you do anything as far as any sort of maintenance or mods right around the time the problem started?
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.