Engine won't crank, nothing!

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Randomly my MDX will not start. When I turn the key on, all the lights, aux features, everything works perfectly except the engine won't start. The starter is not energizing. I can hear relays picking up when I turn the key to start, but no action from engine. I don't have a key indicator either. When this condition occurs, usually we are home. So we wait for a while (twenty minutes, hours, even a day) then try to start up the car. Usually it fires up fine like there was no issue. The dead time seems random in length. Not sure where to go from here. I would take it to the shop, but by the time I would have it towed there it would probably fire up just fine. Anyone have suggestions or previous experience with this kind of problem?