curious infill behaviour

[StevenLessing]

What is this ?
Why is it there?
How can I get rid of it?

It appears when I choose external Fill pattern : rectilinear
Choosing Concentric removes the problem but adds many others in its stead that I don't want to deal with at this time.

On my not so accurate or reliable prusa i3 it causes a lot of vibration and errors on the perimeters.

Please help.

[CompoundCarl]

Pretty hard to tell from an isolated screenshot. It would help a lot more if you posted your factory file and told us what layer to look at.

[dorsai3d]

Those are top solid layers, because your print is tapering inwards enough that the perimeter isn't going to provide the full count of top solid layers. I'd recommend that, if you're having issues with infill like that, you work on the mechanics of the machine, or reduce the acceleration/jerk settings. That will solve the problem at its source.

Or you could go with the true RepRap way™ and apply a bandaid in software. Extra perimeter would help, as would (as you discovered) concentric top infill.