I ran into something similar once with my Taz. It turns out the small extruder gear setscrew had came loose, but just enough so it would randomly slip a little bit, but enough that the main gear was still turning, just not at the rate it was supposed to be going at. Ended up with stringy multi layer prints like the tops of those. An intermittantly failing thermistor or heater core can also produce a similar effect. Check the extruder temperature with a cheap infrared point and shoot thermometer, tighten the setscrew on the motor shaft of the extruder, check for a loose extruder motor plug, tighten the idler arm screws down to 8mm between the washers, and check the idler arm bearing itself to see if it still rotates freely.