If they are lines on the slide film itself they are probably processing lines. I'm having a problem with that with some of the Kodak processing I've been getting lately from the Minneapolis facility.
Processing lines is a result of bad QC at the facility that's processing your film....they are not keeping the rollers in the machine clean.
Where I work, we clean the rollers once a week. I asked a person at a place I used to get my film developed how often they clean their rollers....."Uh, I don't remember" is what he said. I quit going to them immediately.
A noble spirit embiggens the smallest man...