The platform behaviour doesn't seem to like the way you've set your ground tiles. It's getting stuck between the tiles.
EDIT: Just saw Tekniko's response. I guess it's a combination of the two issues.
I recommend two things: first, use the tiled background object instead of individual sprites. Then, for the actual collisions of your player on the solid ground, use a single invisible sprite that's been stretched to fit the space. It can start off as small as a few pixels either way, then just use the handles to stretch it to size.