I'm using my Kossel Mini (R2) to print a part with PETG filament, and i get a problem that at some point. I've also tried PLA and that gives the same problem.

Here is a picture of the printed part:


As you can see, the bottom of the cylinders are not coming out well, and they need to be precise.

I've been watching the printing, and I saw that the trouble is actually started at the edge on the side. It makes two perimeters fine, but to finish the inside it does another line right in the middle of that edge (see red line in picture). At that point the nozzle seems to be touching the perimeters too much, and the printer starts vibrating, which might causes some deviations in the x-y.
Another problem is that it seems to be trying to extrude too much plastic there, there seems to be a pressure build up in the nozzle (I have a bowden extruder), and when it tries to print a layer of the cylinder, it seems the build up pressure is releasing then and I get a very bad cylinder.

I'm not sure how to fix this, has anyone encountered this problem before? Anyone has some ideas on how to fix this problem?

Thanks!