1998 Expedition stalls

For the two months my 1998 Ford Expedition has been stalling about once a week. It will work fine and then I will have trouble starting it. I try starting it and it will turn off right away. Then I try starting it again a second or third time and then it will stay on. This past Friday it did not want to stay on and still won't. The fuel filter, sparkplugs, air filter and the sparkplug boots are new. Anyone have any idea as to what it might be?

Thanks