did you ever try to swap PCM? It sound like you do not get either or both - current for ignition and/or power to crank the engine.
I had similar problem with heat syndrome. When summer came or after long time driving on winter - all gauges stop working. Like I turned off ignition switch, but car was running fine. It turned out to be a dry joint some where on the dashboard. I had to find it by myself.
So, having this problem, I would suspect the same with your car. Try to find PCM module and swap them for a few days. See what happens.
Ignition switch could be also a problem. Replace it too.
But whatever you do, do only one thing at a time.
I just though about other solution.
On the day, when you are SURE it will not start, remove the PCM (ECM, or whatever it is called) and take it with you. Place in a cold place and when about to leave, take it with you, put back and try to start the car.
Of course it is impossible with ignition switch
To narrow down the problem - when the car cranking, but not willing to start, remove a spark plug. See if you can smell gas. In the meantime, check for spark on other cylinder.
This way, we will know if we are talking about fuel or/and spark deficiency.
And one more thing.
Does EVERYTHING else work when the engine does not crank? I mean, wipers, washer, lights, horn, blower, turn signals and so on... including gauges on the dash.