It's really a pretty simple system. The coil only requires a 12 volt power supply from the ignition switch, and a pulsed ground signal from the TFI module. You've verified the power supply to the coil, and you have replaced the coil. This leaves the TFI module. Unlike the coil, TFI modules can work intermittently. A coil is like a light bulb: it either works or it doesn't. If your problem occurs more often when the vehicle is warmed up, like when you go to start it after getting gas, then I am certain the TFI module is your problem.
Take care,
-Chris
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