My 2004 Mercury Sable is having starting issues.

Not being exactly clear as to what the issue you are trying to describe is, all I can say at this point is, did you also replace the spark plugs and wires? What about the ignition coil, have you check and or replaced that as well? It could also be something as simple as a bad ground connection someplace which is an easy fix but not necessarily easy to find. Have you checked all connections, fuses and relays in the engine compartment power box? Did you leave a wire off or a plug disconnected someplace? Last question, but first thing to check: Did you check for any stored trouble codes? If so what exactly are or were they?