I'm new to PICO-8, and my first experiment was to create a simple program that moves a rectangle back and forth. When I run it, the movement will be fairly smooth at first, but then I start to notice a sort of hitching every couple of seconds. I tried this both on my desktop and laptop, and see the same behavior on both computers.

Is this something with my code, my eyes, or PICO-8 itself?

I'm running this on Windows by the way.

Here is the code I'm trying:

x = 109
xs = -1

function _update()
  x = x + xs
  if x < 0 or x > 110 then
     xs = xs * -1
    end
end

function _draw()
  cls()
  rectfill(x,0,x+15,30,12)
end