What has worked for me is to make sure that when AF runs it goes out of focus three times the in-focus value. And I set the backlash comp to stepsize+20%. Works on both my rigs.
If you have step size too big (and on the AF run the out of focus first sample is say 5-10 times the in focus value) the samples are too far apart to find the true focus point. IF you have too big a step size you tend to get a shallow “U” or a bowl shape curve as opposed to a sharp V. But too small a step size and you can fail to get AF to work at all. You just need to play around with this and watch the curves.