Is the line in one place or every time you purl? In other words are you getting those horizontal lines through the piece or in only one place?
I got a line like that once and I was confused but realised I had actually made a small stitch error at the beginning of the line and it simply gave a different structure and look (that came out like a 'line'). It's really only if you're finding many lines occurring that you can perhaps better identify whether its a tension problem or a stitching one.
Lines can also be created if you've left knitting on needles for a long time.