Difficult on start up...weird behavior

[J.Seven]

My 2000 4.4 some times gets very difficult to star up. Batery has lots of power and the engine cranks alot, but it doesn´t start for 30 or 40 seconds. Many times it starts without any problem, while other takes some patient till the engine fires up. It seems it´s not getting gas into the engine, so I´m thinking it must be the gas pump!! Only once the engine dies while I was driving. I thought it could be bad gas, fill it up in another gas station but it remains the same.
What you exoerts have to say about this? Thanks in advance.

J.Seven