Mercedes 6.9 Intermittently Doesn't Start

[Hexaflouride]

Recently my car has been intermittently failing to start. I turn the key to run and lights, power, everything come on like normal, but when I go to crank it, the car doesn't turn over. Not even a click or a groan, nothing at all. When I fiddle with the shifter and move the car though some gears, it eventually starts. This last time, I had the car in drive and it started (is it supposed to be able to do that?). Is this a problem with the starter? I've also read that the ballast resistor can cause starting problems too.