Im trying to diagnose and figure out a steering issue i am having. My 240z is using modified parts so I dont know how much help i can get, but its worth a shot.

Once i start going over 45-50 MPH my steering becomes very hard to keep straight. it darts left and right. i cant tell if its because the road is pulling a tire one way or the other, or something else is causing it.

I am using an OEM steering rack, Apex Engeneered control arms and tie rods. I also have BC coilovers with the suspension low. I also do have a negative camber to fit my wheels which are 18x9.5.

Everything that i can tell is tight alignment has not been done by a shop but I squared it up the best i can to the body within a 1° or so.

Check the bushings on the steering rack cross member mount. Sounds like the rack is shifting left and right as you drive to get those large direction changes.

There is a difference in the early and late racks where the bushing is smaller (narrower) than the later racks. If you have a late rack and early bushing, it might be the cause of your woes. Or the bushings are just shot. Place to start any way.