Did you ever figure out what caused this?
I had (have?) the same problem with my 2003 Explorer. Fuel pressure issues, which the mechanic diagnosed as a bad fuel pump. They replaced the fuel pump and filter, and I then also replaced all the injectors.
Worked fine for a while, then after a couple of weeks the issues returned, eventually leading to lean codes.
I took the car back to the mechanic, and he said that the fuel pressure damper was bad, which had caused the fuel pump to go bad. I'm not sure whether that's even possible. Plus, the damper is one of the first things I checked when this issue first came up, and I didn't see any of the telltale signs of it being broken. Anyway, I only just got the car back, so I haven't driven it enough yet to know for sure whether the problem is fixed for real this time,