1999 b2500 cranks, sparks but will not start I leave the truck alone for a few weeks and then it starts

SOURCE: 2000 F 250 WILL NOT START CRANKS OVER ONLY.

you should have the pcm scanned for dtc's... probably a bad crank or cam sensor

SOURCE: Ford Explorer 97 - cranks but won't start sometimes

I don't see anywhere where you replaced the drive belt or the fuel filter it could be building up pressure in the fuel pump causing the sending unit to stick but before you do all this get a digital volt meter and check out the crank sensor I'm just asking because it has happened to me if you have a lot of keys on one key ring it can damage the ignition switch so check out the crank sensor and the ignition switch first

SOURCE: '94 Ford Taurus Wagon 3.0L crank won't start

Sounds like fuel delivery, Maybe fuel pump.

SOURCE: 1997 Ford Explorer - I can hear it crank, but won't start

try tapping the crank sensor while someone turns the key on(dont try to start just turn key forward until the radio will come on) it should whirl when the key is turned, if not tap it then you should hear it whirl. then try to start.

SOURCE: no spark 1991 ford f-150 5.0 EFI

check your eec wires on driver side under hood for shorts