Had a similar problem when I set up my new router. Checked all mechanical components, made sure no slippage anywhere. Still lost z position. Not sure how your drivers are connected but I would double-check polarity of step signals between BOB and driver. When you changed the driver, did you also have to make any changes in Mach3 (or other motion control software) - like step signal polarity or z direction? That was my problem - driver is triggering on wrong edge of step pulse and the end result is that you lose one step every time you change direction. Doesn't really show up on a simple xy profile cut but once you get a lot of z movement, that's when you see the effects. If that is the problem, fix is very easy.

So, how's the driver connected, and did you change anything when you swapped drivers? What were old and new drivers?