96 F-150 starting problems

my sister has a 96 f-150 and it wont start. Its not getting spark out of the coil. we have replaced the coil 2 times and also the cap, rotor, plugs, wires, and the ignition control module. we have power to the coil bit not coming out of the terminals. all the sudden one morning it woulnd't start, so i checked spark out of a spark plug wire and found it had no spark, i went to the coil wire and had spark so i took the cap off and replaced the cap and rotor. then it started and ran fine all day long. the next morning it wouldn't start. so i tested the coil and had no spark. we replaced the coil and still had no spark. so we replaced the plugs and wires. still no spark. so we got another coil still no spark. then we got the ignition control module and still no spark. i told her to call a mobile mechanic. she called him and the next day he came to her house jumped in and the truck started up fine. now 3 days later no spark again even though the diagnostic was fine when the mechanic checked the truck
PLEASE any feedback would be appreciated.

any luck i am having a problem like yours?

When the vehicle is having the no start problem and you have no spark out of the coil use a 12 volt test light to see if you have 12 volts at the positive side of the coil. Then place the coil at the negative side of the coil, have someone crank the engine, the test light should flash. If the test light flashes it means the ignition module is grounding the coil to tell the coil to spark. If it does not flash it means the ignition module is not telling the coil to flash. If the ignition module does not tell the coil to flash it could be the ignition module, or it could be whatever the module uses as a triggering device. It could be a crankshaft positioning sensor.