Inconsistent bumpiness

[eigenbot]

I've been printing a lot of the same item lately. The outside is a scalloped edge. Sometimes the layers print quite smoothly, but sometimes they have odd bumps around the edge. Sometimes the first several layers will be smooth, but the last several will be bumpy. I've attached pictures of all 3 cases. The picture seems to be loading upside down, but the white piece (on the bottom) was smooth for the first several layers but bumpy on the last (the side touching the table, you can see on the side where it's not as bright). The next black piece is all smooth, and the black piece on top of the stack (bottom of photo) is all bumpy. The filament in the pictures are the same brand in different colors (Hatchbox PLA), but I've had it happen in other filament brands too. Not sure what might be causing this uneveness since in multiple prints with the same settings, it happens in some but not others. Sometimes I'll print something else inside, but I haven't noticed a correlation between the bumpiness and whether or not I'm printing multiple objects. Other items I've printed with this filament has never seemed to have these problems. Any ideas?