1999 Mercury Mountaineer Cranks, but won't start

Starts about 5 out of 6 times. Will always turn over. Once started, runs very smoothly, idles fine and runs fine at speed. When it won't start, we give it a few minutes and most of the time starts within 10 minutes (with various numbers of trys). Have replaced the mass air flow sensor. Does not appear to be related to outside temperature or humidity, as it has started during rain storms and when it's been very cold. Local dealer said he could not repair it unless it wouldn't start for him. And it started flawlessly for 3 days.