I used to have problems with the spool on my SD4 occasionally going haywire and if I don't catch it in time it ruins the print.  Last week I tried a quick fix and it works phenomenally.  Just drill two holes in the PVC pipe, bend some wire around a pencil a couple times and poke it through the holes.  Not only have I never had a problem - even with full spools, but my prints are smoother now.  Apparently, even when it doesn't completely snag the variation in friction was causing slight modulations to the print.  For my narrow spools, I have some foam spacer I use to keep the spool centered.