Overlay image and smoothness

I gathered some images from internet (in some dimension) and concatenated in a file. You may find this file here: https://s13.postimg.org/quxuzstef/catalog2.jpg

I am trying to make a sliding show effect by using ffmpeg's overlay option. The command i use basically creates a window and move an image beneath this window. Here is the command i use (drawtext part is optional)

ffmpeg -r 25 -loop 1 -i catalog2.jpg -filter_complex "\
nullsrc=size=800x320 [background]; \
[0:v] setpts=PTS-STARTPTS [tmp_catalog_file]; \
[background][tmp_catalog_file] overlay=shortest=1:x='-n*4' [out]; \
[out] drawtext=fontfile=Arial.ttf: text=%{n}: x=(w-tw)/2: y=h-(2*lh): fontcolor=white: box=1: boxcolor=0x00000099
" -f lavfi -c:v libx264 -f mp4 -t 20 -crf 0 -y out.x264.mp4

The video, for a reason, stops for a frame (or a second) and breaks smoothness at random points. I tried using python PIL and opencv2 libraries together to make a video, which gave better result but didn't eliminate this choppy effect. What could possibly be causing this?