Looking at those photos it looks like the only consistently good walls I'm getting are at 3mm.
I watched it print that piece for a bit, and noticed the grinding/skip sound from the feeder about once per revolution once it was above 3mm. It was a short sound, but pretty consistent per layer.
It feels like there is some fundamental design issue with the UM2. My old Thing-O-Matic was a bit long in the tooth after 2 1/2 years, but I never had any issues with extrusion, even at fairly high acceleration rates.