Thats the funny thing about coils, TFI and such. They can produce a sporadic problem that when tested results in a positive test. But still can give you problems,.. sporadically. Later 4.6 cars are notorious for the COPs causing sporadic issues. I even had a hairline crack in on of the modules on my 96 causing a misfire code but I couldnt see it and the module would test OK when I checked it. Electronics are the most frustrating portion of an engine to me. Because you can spend hours,.. sometimes days trying to chase down the root of a problem.

A bad pickup coil can cause something as minor as bad fuel economy to not working at all (Engine failure,.. no start) and everything in between. Hard starting is one of the symptoms of a bad Hall Effect sensor. Some other symptoms can be missfiring at either higher RPM or at low RPM under load, a hesitation or stumble when you step on the gas. Constant misfiring, low on power etc. My father inlaw told me once that if the plugs turn sooty for no reason its usually a problem with the coil,.. low voltage or wiring issue, arcing in the distributor cause damage to other parts inside it. In your case you could have a faulty Hall Effect sensor. The Hall effect sensor also tells the computer which injector to fire and for how long. If you probe the #1 terminal on it the test light should flash when cranking over the engine. If it doesnt,.. well the engine shouldnt be running. Other then that I dont know of any other way to test it.

Here is a good link that should help you.

SBFTech.com TFI Module/Hall Sensor Testing Procedure

Hurst