I don't think it's your neutral saftey switch. The NSS will not let it crank unless it's happy. Another way to test it is the next time it happens take the trans from Park to Neutral. If it starts it could be the NSS. It could also be a sticking relay.

We all have been chasing this problem. Mostly 4.4's though.

First thing you need to do is see if you have any codes. Go to Auto Zone they will read you OBDII codes for free. Any of the sensors or rheostats in the system will throw a code. None of the relays and most of the switches won't. A wiring problem is anybody's guess. Then go back to page one of this thread and start reading.

I'm working on a post that will recap everything we have all done. I won't be able to post it until later this week.