Sewing Machine Adding Zig Zag Stitches to Straight Sewing?

I feel as though I've looked everywhere for a solution to this problem with no luck. I'm hoping someone here might have an idea. When I set my sewing machine to zigzag stitches, it works like a charm. When I try to sew a regular straight stitch though, the machine will stitch fine for a while then kick a single stitch out like it's part of a zigzag. The odd stitch comes along regularly, not randomly. I've fiddled with settings and I'm sure I have it threaded properly.

Anyone have any idea what might be going on?

Thanks in advance!