Extreme underextrusion. Lots of things can cause that. As you can see from the replies you've already got, people are just throwing out ideas drawing from their own varied personal experiences. You're probably going to have to try them all, unless you get lucky and identify the problem right away.

Make a list of all possible causes of underextrusion, try to order it from most to least likely (as best you can), and just start exhaustively checking each and every possibility.

To get you started:

• Print speed too high
• Nozzle temp too low (210 C is about right for PLA, but too low for most other types of filament)
• Hot end assembled incorrectly (e.g., PTFE tube not seated properly)
• Heat creep
• Poor quality or dirty filament
• Extruder not gripping filament properly
• Extruder gear slipping on stepper motor shaft

Less likely, as they would result in consistent underextrusion (yours seems inconsistent to me)

• Flow too low
• Esteps not set correctly

To help diagnose, watch your extruder motor, gears, and filament during a problematic print. The way it behaves can help you rule some things out (eg, you'll be able to see if the gears are slipping on the motor shaft, or if the motor is stalling, or if it's just grinding away at the filament without feeding it through.