As Art suggested it might be the Crank Position Sensor. Look for worn/damaged wiring and wiring sheath. I don't know the specifics as I've never had to replace one but there is an electrical test you can do on the CPS. Try a search here for instructions.
Since it is an intermittent problem I suggest also checking the underhood 25 amp blade fuse for the fuel injection/ignition. I had an intermittent starting problem with my 90 240 years ago. After struggling mightily to find it, I finally discovered it was a corroded fuse holder for the 25 amp fuse. Replaced the fuseholder, the 25 amp fuse, cleaned up the wiring and I was good to go... If you haven't replaced that fuse and fuseholder I would check them ASAP before throwing any more parts at it...:)
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