Random under-extrusion on first layer(s)

[duhanebel 0]

I've been printing for a while with a Creality CR-10S with Colorfabb black PETG economy and all was good. Until today, where on a large, flat print (30x10cm - a small drawer) I can't manage to get a smooth first (or second) layer no matter how hard I try. I tried so far:

• calibrating the extruder
• setting different temperatures (range 230-240 suggested by the manufacture is 235-250)
• slowing down the print (I usually print skin at 80mm/s, I tried this one down to 40mm/s)
• installing a brand new nozzle
• levelling the bed

The print starts fine but then, randomly, some bands that look like under extrusion appear: 

 

Here's a second example of the first layer - I've ripped it off the plate but as you can see the first 50% of it was fine. The problem appeared later in the process. 

 

 

The extruder "clicks", the filament stalls and it's clear that the nozzle is not extruding plastic fast enough. What can it be?

All other prints are fine, but I have to say this is the only one that has such a large horizontal surface.

 

What can it be?