You have four frames that look very similar where one leg passes over the other.
During those frames the foot in the air travels a very short distance.
I have some of that problem in my edit as well.
This is a recolor of your latest run cycle with some trace data.
From these traces you can see that the foot travels at different speeds on the ground too.
Just move frames 2 and 6 back one pixel and it looks much better.
The arm movement is pretty good now, probably better than the one in my 12frame silhouette.
In frames 3 and 7 I guessed where you have the hidden arm. I'm not sure that was where you intended it to be.
In my version I'm hanging a little longer on the arm movement when the arms change direction but I'm not sure that's better.
EDIT:
here's your and mine next to each other
It's obvious my version has a few issues. I've messed up the speed of the foot movement as well.
And nothing really moves in a smooth path and I have no idea of how I was thinking about the arm movement.
I
wll might make an new edit, 8 frames full color, that I can feel happy about.