RE: Horizontal print lines
While this *might* be a slipping grub screw on one or both belt drive pulleys it doesn't look bad enough; so it's probably dirty smooth rods or bad belt tension.
First, with the power off, move each axis by hand; is it smooth? Does the resistance stay the same over the whole range?
Clean and lubricate the smooth rods, check they are not damaged/scratched. Check the idler pulley is not clogged, out of line, or damaged (loosen the belt and try turning it with your fingertip, is it smooth?)
With the belt loose try turning the stepper motor by hand; is it smooth?
Retighten the belt until it just begins to make a low musical note when plucked...
Make your next print this:
https://www.prusaprinters.org/prints/46639-tension-meter-for-the-gt2-belts-of-i3-mk3s-or-prus
It MUST be printed in Prusament PETG or the spring tension will be wrong.
Let us know how you get on.
Cheerio,
RE: Horizontal print lines
Did you solve this problem?