I may have some bad news, the 98.5 mercury mystique I just traded started doint the same thing. It would not start every once in a while. The starter motor was running, but it would not start. I took it to the Mercury dealer, and they replaced the starter, it still would not start all the time. A week later when it would not start at all, I took it back. They looked at the flywheel, (the gear that the starter turns to start the engine). The flywheel was warped like a potato chip. If your starter motor is running when you turn the key, and it won't start the flywheel is most likely bad. For a few months in 98 ford used a cheap flywheel, and your car might have one. I know about this because my dad worked for ford. Take it to the dealer and insist that they look at the flywheel. Don't let them tell you that the starter needs to be replaced again. They will most likely find that the flywheel is bent, have them replace it. It will take 3 or more days, depending on your dealer. Also insist that they make ford pay for it. The flywheel on my car went bad at 62 thousand miles, and ford replaced it for free. BTW my car first started doing this on Dec 21, 2000 the first day it got below 15 F in Ohio, so this is very likely to be your problem.
