Based on what's online (YT etc):
- ECM Relay: Changed both (ECM/Main and Start 30A relays) with new ones
- Fuse box/wiring corrosion: Mechanic checked and they are good
- ODB2 check reported no errors
We had the fuel pump, injectors, fuel line, battery, starter, camshaft sensors, MAP sensor, O2 sensor, air and fuel filter all checked (some even replaced) and there's been no issues with them.
At this point I'm stumped. Any clue.pointers?
Thanks much and have a great day
Does it start in morning then won't start after. Will it start if it sits for a while
Thanks for your reply @Nissanrob88
Yes an no...
it starts fine many times in a row then "out of the blue" seems to never want to start again. That was why I thought perhaps a the 30 A "Main" relay ( the one next to the "Start" relay) could be faulty or work intermittently... Since the "Start" always worked (as the starter was always cranking fine) I tried and swapped them... so I put in 2 new relays to chalk-off the possibility of a faulty relay... it seemed to work for 1/2 day, then same problem... car would crank but not start once again. 
The tech guy in the shop tried to no avail checking al onboard computers, from the "code" one to the immobilizer, the engine module, the key module and so on. He even swapped in working module (ours is a 2005 with code 0281011694) and uploaded the code to flash the eeprom still nothing. 
This ha become an enigma wrapped in puzzle, boxed in a mystery, rolled in a conundrum 
I can only think of 5 things. One of the pins on main harness to computer has a short. The crank sensor is going bad or the harness is going bad. The neural safety switch has short or is going bad. It's really hard to say. Something like this needs a good scan tool that can look at all modules, sensor ECT ECT. Its also possible to have a intermittent grounding issue