I have a 5.0 that runs great many days, except--- sometimes the only spark it has is when I turn the key on or off the key, essentially making and breaking current to the coil. But while cranking there is no spark. One day it performs flawelssly but the next day it does as I have mentioned. It quit cranking at all while loaned to family, and I replaced the ECM mounted next to the radiator, then it ran perfectly for weeks. I have found that it will start if I turn the ignition key on (not to start) and then off about five times and then turn it to crank. Any help appreciated.

 

Something you might try.
Connect a test lite or voltmeter to the coil terminal that is fed power from the ignition switch. Run the wires to the inside of the vehicle and any time there is power to the coil the light should be on or 12 volts shown on the voltmeter. If the ignition switch has a problem supplying power to the coil the light / meter will show that.
Did you have a chance to check the battery cables for corrosion or possible poor connections?