I agree that you need to make sure you have AFMA set for your lens. I would also recommend learning a new way to focus your shots since the 1DX has a ton of settings when it comes to auto focus and using one shot is probably not the best in many situations. I used to use one shot quite a bit, but I almost never use it now and the 1DX rewards me with about a 98% hit rate even with some very difficult shots at 85 f/1.2 or 200 f/2. That hit rate is after approximately 100K shots on my 1DX. Yes, I shoot quite a bit too.
These videos might help understand the complex auto focus system on the 1DX. I am still trying to learn what settings are best for each type of situation. Get it right and you should get killer sharp images.http://learn.usa.canon.com/galleries/galleries/tutorials/eos_1dx_tutorials.shtml
If you use a tool like FoCal then you can test auto focus consistency which should help you understand what focus points are more consistent and if your lens is working properly.