1998 Ford Explorer Intermittant startups

Engine Performance problem
1998 Ford Explorer 6 cyl Four Wheel Drive 92000 miles

I've had this Eddie Bauer Edition for over seven years and have never had any problems, however, about six months ago when I turned it off for about 30 min. It would crank but wouldn't start, then after about 20 min it started. This has been happening more frequently over this period of time, I checked all the fuses, especially the fuel pump fuse, and banged on the fuel pump relay to no avail.
When it just happened to not start near an auto repair shop I had them pull it in, explaining how it only started when it wanted to. The next day they had it ready saying it was a fuel pump relay ($54)
4 days later, same thing. No start, even when cold, then it starts after a few min. I installed another new fuel pump relay also, didn't help
A friend who had a ford ranger p/u said he had the same problem and replaced the DIS module, I replaced the DIS and it started right up.2 days later, same thing happened.
sometimes you could hear the fuel pump running after turning off the motor and sometimes you hear it when you turn on the key and leave it on a few min before cranking, but not always. Now I don't hear it at all because it's not starting at all.
My question is, does a fuel pump react this way by going bad gradually? A Sheriff's Dept mechanic who worked on our Crown Vics for years says no.
What else would cause it to act in this manner and then finally quit alltogether?