I think I may have discovered my problem. First, the camera is affected by the service advisory, so I'll probably need to send it in anyway.
But, here is what I've found:
I was testing the AF with my 3 AF lenses (Sigma 35, 100-400, and 500 f/4). The Sigma was focusing very erratically and not at all near the mfd. And the 500 wasn't focusing at all. But, the 100-400 seemed OK. So, I also tried the Sigma on my 5D III and got much the same bad behavior.
But, why wouldn't the 500mm focus at all? Then it hit me...I was testing the 500mm with the 2x III attached. Removing the 2x and using the lens bare or with the 1.4x seems to work fine. Then I looked and the firmware on the 1DX doesn't appear to have ever been updated. It was only a recent firmware update that allowed the 1DX and 5D III to AF with f/8 lenses. Updating the firmware will probably put things right.
My sincere apologies Clemson656 for being a dummy, but it's taken me all evening to understand why my new-to-me 1DX was having AF issues. I'm working on the firmware update and I'll report back. I'm optimistic.
So glad to hear that you figured out the problem.
I didn't update the firmware as I've heard people having issues with it. So, if it ain't broke, don't fix it, right?
Anyway, Canon will take care of it free of charge if you want to send it in. Just give them a call.
Enjoy the camera. I'll miss it for sure. Cheers.
P.S. Canon would send a shipping label for this issue with 1DX, but they refused to provide a free shipping label if I want to send my 200/2L in for the issue it has with my 5D3.