It's not an PCM / ECU issue.
For a no start, no crank condition, check the following:
PATS (Passive Anti Theft System) is behaving normally.
The battery is in a good state of charge and battery terminals/cables are tight and corrosion free.
The starter motor relay is good. Swap it out.
The clutch pedal position sensor or neutral safety switch is functioning properly. Bridge it with a jumper.
The ignition switch is good. Test with a multimeter.
The starter motor itself tests OK. Inspect for loose connections/cables first.
The wiring associated with the starter circuit is intact. Multimeter and circuit diagrams again.
For an intermittent condition like yours, you have to be methodical and check each component.
If I were to suggest a quick check, I'd say check the clutch position sensor (if it's a MTX) first as the pedal neutral switch does get stuck.
Other than that, the next time it fails to start, you need to check the starter circuit as to where the voltage is being interrupted.
edit: whynot beat me to it.