Infiniti G37S 2008 pulling both ways

Sometimes when I drive my Infiniti at 50 mph or higher it pulls both ways. I have to hold on to the steering wheel really hard to keep it going straight. This does not happen always. The dealer could not replicate the problem and returned the car without any repair.

How many miles do you have on the odometer?
How many miles do you have on the tires?
Do you manually check your tire pressure and correct it as necessary?
Have the tires been rotated, as per the maintenance schedule?
What about alignment? When did you last have alignment done on the car?

You said sometimes. What conditions does it do this? Is it possible that it occurs only on grooved pavement? If so, then that is groove wander and while it is annoying, it’s just the tires trying to follow the grooves in the pavement. A different tire - meaning make and model - would avoid this.

But if it isn’t grooved pavement, (or ruts in the road), then we’ll need a better description.

Have you recently changed the tires on your G37 Sport Coupe?

The Infiniti G37 vehicles (both coupe and sedan) are extremely sensitive to the tires used.
So much so that Infiniti only recommends using Directional tires on them, and never uni-directional tires (tires that point one way on the right side, and the other way on the left side). These tires typically say “Outside” on the sidewall. Directional tires usually have a direction arrow on the sidewall, in comparison.

Check the tires, check the tire pressures, and have the alignment checked if the tires are in fact directional tires…