After changing bits and probing Z, the Z is way off?

I am having much the same problem but only with my Z. The G code has been checked and it is ok. The problem is not every time I run the program. I thought that the issue had been resolved…then it comes back. I can’t trust it to run correctly. I can’t afford the use wood to be used for a client so I have been using scrap, MDF and even hard foam insulation board. I will b I have done a visual inspection of the Z cable and it “appears” fine. If I can’t somehow get the problem resolved soon I may just sell the machine and try something different. Obviously I’m frustrated. Please, one of you really smart guys help!!!