Bad printing layers

[jonbonne 0]

So I've had a small issue just recently with my Ultimaker Original+ printer. I'm printing with ABS at ~240C and heated bed at 100C. The print starts fine and goes well until about ~3-4 centimeters up. At this point, it looks as if the nozzle is clogged or something as the material begins extruding non-consistently and spit-sputter stringy... but after about 4-5mm of this behavior the extrusion returns to normal, the nozzle begins extruding smooth and consistent again and the rest of the print completes satisfactory as long as the print-product doesn't break at those weak layers (which it usually does).

My questions are what are the causes of this phenomena? Why would the nozzle behave like it's clogged 4-5mm consistently at around 3-4cm above the build plate then just return to normal operation? Would insulating the print volume help this? Any other suggestions or pointers?

Thanks!