I have noticed a few threads around the internet with people having issues with their Z axis adding steps and incrementally lowering. Many of them assume that the inertia of lowering the Z plate and spindle is causing the incremental drop. I am having a similar situation, but running some calculations, my stepper motor should be plenty powerful to handle the inertial force.

While running a project with numerous z movements, it would get incrementally lower and I would have to stop the program, re-zero the z to the top of the work surface and then continue from the next line of code. However, I noticed this happening even with shorter v bit runs. A recent sign project I was doing ran a simple flourish on the left side of the date "2016" and then ran the mirrored flourish on the right. But the mirrored flourish was deeper into the project wood. somewhere moving from left to right pattern it had added some steps.

I ran some simple math on my Z plate weight with spindle, linear blocks, and spindle mount. The combined weight is around 25Lbs. I am running a NEMA 24 3.1Nm stepper parallel to a 1605 ballscrew with a 1:1 belt at the top. one of the online calculators I used said it should only take around .120 Nm to lift that amount.

Here's a link to a video running my router bed. In the middle of the bed you may notice i started tracing my tool path as my initial .2" clearance had dwindled down to below the top of my work piece.
https://www.youtube.com/watch?v=A280CtS-_4c

my setup:
Mach4 on Windows 7 pro 64bit
ESS connected to generic BoB
NEMA24 3.1Nm Stepper motor
M542T Stepper Driver powered by 36VDC
Z Driver set to 4 Microstep for 800 steps per revolution
Mach4 motor tuning set to counts/unit: 4064  velocity/unit: 100  accel: 5.00

any help would be much appreciated.