When it's happening[not cranking], drop the steering column covers to get at the ign. switch wiring. Jump the main hot wire and the starter input wire terminal[power that goes out from the switch to the starter motor] If the starter now merrily cranks away, then your ign.switch is acting up intermittently. Cheap part, easy to change, and you don't need to mess with the ign. lock cylinder, just replace the electrical portion.
If you have an automatic Montero, it could be the Park/neutral switch. Even if it's in adjustment, they DO go bad. I just replaced mine a few weeks ago for a no crank problem. I got one shot trying to start the truck in Neutral instead of Park, it did start, but then the switch died completely at the next time I needed to start the truck later that day, with neither Park or Neutral working.