I have been using my x-carve for a couple of weeks now with great success. Recently I was making a picture frame and cut out the area for the picture. The cutout has a slight wave in it (about .020). I have tried this in several different areas and it doesn’t matter where the cut is it’s always along the x axis.I have checked all the v-wheels and everything looks good and tight. I placed a v-bit in it and lined it up with a line on the board and moved the x from far right to far left in 1 inch increments looking for a variance off the line but could not see any. I have attached some pictures of a cut from this morning, it may be hard to see. I have a Dewalt 611 router and using a 1/8" 2 flute down cut bit. If anyone has any ideas please let me know.
You can see in the second picture it even changes the depth of cut like the whole carriage is rocking.
Is your machine size 1000x1000 mm? If so you may need to stiffen your X axis to help support the heavier Dewalt router.
Yes it is 1000mm and I did the stiffen mod when I originally put it together.
Ok, so much for the easy answer 
Move the router all the way to the left and feel the top and bottom of the X axis rails. Maybe get a straight ruler and hold it along the V profiles looking for some sort of slight deformation. Do the same thing on the Y axis.
May also want to inspect your V-wheels to see if they have any kind of deformity or crud stuck in them that could potentially change something.
Finally, try to make a straight X axis cut in something with consistent material properties, like MDF. Maybe the bit got drawn out of line by an extra-soft or hard section of grain?
Also try a really sharp, new bit in the same material. Down cut bits exert a lot of force pushing the bit out of the material, especially when they begin to wear. Tighten your eccentric nuts on the bottom your your spindle carriage even tighter as well. I bet it goes away with sharp bit and tighter v-wheels.
I have checked all my wheel and everything looks good and is tight. I have had the same problem with mdf and pine. Both were with a down cut bit. I am going to try playing with a regular cut bit tomorrow morning and see what it does.