My Ultimaker 2+ started showing this exact symptom a couple of weeks ago. At first I thought it was a firmware problem caused by static discharge, but after reloading the software the problem continued. I eventually realized the problem occurs only when I put my new thermal cover on the machine for use when printing ABS (which does help the print quality immensely!). Without the cover on for PLA printing, there was no problem. I had a spare Rotron blower that happened to fit under the printer, and blows through the main electronics enclosure. It has printed several ABS parts for the last 40 hours or so with no further problems, so I think it was heat related and the fan fixed it.