Curling corners and irregular edges with ABS

Hi all,

A newbie here. I borrowed a Prusa i3 MK2 from a colleague two days ago and went head-first into 3d printing.
Now i have hit an issue that I see no improvement in after 4 or 5 reprints an alot of googling and tinkering in between. So I need some help

I am printing a classic Lego brick (4x2) in ABS and I keep getting curling corners and collapsing vertical walls.
What am I using;
- Prusa i3 MK2
- ABS 1.75mm RAL0 (whitish) from 3d4makers
- Slic3r software

After 2 initial prints without paying much attention I did the following set of prints

The first print was done with default setting from Slic3r for ABS (nozzle 255/ bed 100 / 50 mm/s perimeters)
Attachment 11929


The second try Slic3r settings were the same, but during printing I lowered nozzle temp from 255 to 240 and again to 230 at 3/4 of the print
Attachment 11930

Third and last print I set nozzle temp to 230 in Slic3r and set extrusion multiplier to 0.8 (was 1.0). Also I set printing speed of perimeters from 50 mm/s to 25 mm/s (external perimeters also adjusted to 80% of perimeters)
Attachment 11931

I didnt use brim or raft on any of the prints. Infill was used in all 3.

There are some differences in result between prinst, but they are not obvious and there are certainly no obvious improvements.
So, crowd, any suggestions please?

Thanks a lot!

Remvs
The Netherlands