Strange layer problem  

Strange layer problem

Hello,

I've put this printer together without problems thanks to the very good manual (and candy 😀 )
Calibration was no problem, only had some trouble getting the first layer to stick in the calibration.

But when I print larger items I get the following problem (I tried googling for it but I don't know if there's a specific term that might be useful to search with)

I've tried setting the extrusion rate to 105% but no difference (in fact it seems to have made it worse), I've tried both the slic3r and prusa control software, no luck. All other settings default, fine detail

The first time I thought it might be some temperature or draft so I put the printer in the heating room in the basement with no change of draft or temperature changes. No luck.
What I do notice is that my first layer stick very strongly to the metal sheet and I have to pry it loose, often separating it from the rest of the model;

Any hints or models I can use to calibrate would be greatly appreciated (this one is a bit big for calibration as it prints 2h30)

Thanks in advance!