FP2 X traversal suddenly dropping by .04mm

I took some of my first test cuts with my FP2 and I noticed that the Z is dropping when traversing the X axis. In a 20mm zone the Z drops by 0.04mm, and then it is a linear climb to ~.07mm over the 400m X axis. Traversing the opposite direction the Z shows the same symptoms albeit in a slightly different place along the X.

I noticed the Z dropping with my DRO. It drops suddenly from 0.006mm to 0.04mm at a set point along the X. I figured something must have been up with my DRO. I setup my indicator and checked the knee and it is dropping by the same amount the DRO is reporting.

Feeling a bit disheartened given the work I have put into this machine I started checking the obvious:
- Locked the Z and traversed the X: No difference (same deviation as mentioned above)
- Tightened the Z gib: No difference
- Tightened the X gibs: No difference
- Removed the top flat X gib: No difference
- Removed the tapered X gib: No difference
- Backed off X leadscrew lock nuts (thrust bearing): No difference
- Backed off the X leadscrew nut: No difference
- Tightened both the X and Z sliding plates (to box ways): No difference

I am just about out of ideas. Any thoughts on what I should check next?