Remove the extractor and then use a dremel to slightly bevel it on the bottom. They're just too agressive. Once I did this with my Witness', they ran fine.
Since I'm kind of out of ideas beyond trying to reshape the feed lips (always a sign of total desperation) I looked into your suggestion about the extractor despite the fact that if you look at my photo of the failure (typical of the only failures, all are like this) the nose of the round is well above the top of the chamber centerline.
I assembled the gun without the recoil spring, and slow cycling dummy rounds I find they snap up under the ejector smartly and the rounds enter the chamber with only a slight bump on the feed ramp. Basically if the round is held by the feed lips at all parallel to them, I don't see how its possible bullet nose can get that high and how inadaquate extractor clearance could do anything but cause a failure to go into battery hang up (with the bullet having partially entered the chamber).
I've taken my worst mag (which will have this failure almost every time) and crushed in the body at the top of the recessed area right behind the slide lock notch, to move the feed lips together slightly. This of course makes it not lock back so I filed the sides of the follower until it came up enough to lock back again.
So next time out I'll see if this mag is better or not and will pay more attension to the relationship of the extractor and the jammed cartridge.
Normally I've an very low opinion of "recoil buffers" but since a stronger recoil spring (and stronger mag sprnigs) appear to have greatly reduced the frequency of the failure, I might consider trying a recoil buffer. Problem is, I don't see enough extra rearward movement of the slide for there to be space for one -- with the recoil spring out, I could easily examine this in great detail.
Could you post a photo or a link to where you got your Witness recoil buffer?
Edit: Found this via Google,
http://www.buffertech.com/stores/1/CZ-75_85_Recoil_Buffer_P9C2.cfm is this what you use?
--wally.