What can cause a sudden and dramatic loss in the inter-layer registration of my prints?
There are many factors, here are a few things to check:

I'd first suspect filament feeding.  This type of ridging can be caused by a filament coil that is binding occasionally, or a filament that doesn't have an even diameter or volume per length.  Binding within the filament feeder and feeder tubes can also be a cause. Bubbles in the filament, or sometimes a mismatch between the filament ideal temperature and the head temperature could create results like this, but it probably wouldn't vary so much between the layers.

Next I'd look at the print head.  If it has blockages, or poor temperature control this could result.

Lastly, I'd check the mechanisms - disconnect the motors and see if all the carriages slide smoothly without any binding, particularly the Z axis.  It doesn't look like you're missing steps, but binding here may result in greater backlash, which could result in similar ridges. Make sure any belts and gears are tight.