Intermittent Extrusion Issue with FolgerTech Kossel Delta and CuraEngine

I'm looking for troubleshooting advice on intermittent extrusion issues with my FolgerTech Kossel Delta printer.

I am printing a 1.2mm walled structure (a pod for an R/C glider) Using Repetier v2.1.6 with Cura v4.6.1, using a .4mm nozzle with PLA filament, heated bed and an extruder temperature of 180 degrees (that has proved reliable with PLA in the past). Cura configuration, STL and GCode files are attached in the hopes those show a smoking gun of some sorts.

The issue: in the middle of a print, I will intermittently experience "clumping" of the extruded filament like shown in the images below:

Attachment 14711 Attachment 14712

Sometimes it will happen early in the print. Other times it doesn't show up until the last 50 or so layers.

I have checked the following:

• The heating element and thermistor are securely attached to the heater block.
• The heater profiles show normal heating and cooling cycling, but the average temperature stays consistent even when the clumping occurs.
• The Bowden extruder seems to be extruding normally (i.e. filament does not appear to be clogging).
• I am printing at the slowest speed available in Repetier Host to preserve detail.

Any thoughts would be appreciated.