I'm sure you're looking at the right place. Basically with pixel rounding, since it's always moving forward, if it gets in a spot where the x is actually between pixels it will move a pixel ahead or back even if the controls aren't touched since the speed isn't exactly 1 pixel per tick. Pixel rounding is doing exactly what it's supposed to do, and the fact that you're not noticing might mean it's pretty trivial, and no one else will notice either :P . As far as I know there's no way to prevent this besides redesigning how the level moves.