Cura Wall random bumps

[ArtyMarty 0]

Cura doing weird things...
Can anyone tell me why this happens?? I've sliced this with various settings on wall/shell thickness , even different overall resolution. All with the same result.
It basically does weird bumpy stuff on a section of layers . if you look at the zoomed out view you can see the green of the inner wall showing through. there is nothing unusual on the inside of the print causing this as far as I can tell.
I even upgraded from an old version of cura to the latest.
I actually did the realtime Play version of the layers to see what it was doing and you can see in that screenshot it's not a "starting a new layer" problem as it's basically in the middle of printing the layer. It just does that outer wall a bit further in and then makes bumpy bits. The outside is a continuous steady angle. It's fine at the bottom, does a few weird mm of layers and then goes back to smooth/normal/nice.

[gr5 1,270]

I'm pretty sure this is an issue with your STL file.  Did you create this model?  

 

If you did you probably want to reduce the qty of triangles created when you exported to STL.  When the triangles get smaller than around .1mm you start to get stuff like this.

 

If you didn't create this then I would pass this model through meshlab and remesh it with quadratic decimation.  It's very easy to do - one menu option possibly cut triangles in half or 1/4 and then click apply.  More here:

http://www.shapeways.com/tutorials/polygon_reduction_with_meshlab

 

By the way I'm pretty sure you don't want support for this part - it's more trouble than it's worth.  FFF printers can "bridge" just fine.  The only overhangs you have are all bridged (supported on at least 2 sides).