What causes a 1997 ford probe with the 2.0 Mazda engine to misfire?

I recently bought a 94' GT...which at the beginning would run but would not idle...and when we did some work on it and got it somewhat stay running...we drove it and it misfired. We were looking for a cheap fix because at the time we hadn't bought it yet. So we changed the spark plugs...but that wasnt the problem. So the next step we changed the Ignition wires and it purred like a kitten. the problem was that the wires had never been changed since the car was manufactured and were getting worn out.