Zigzag issues

Question

I have a Necchi 3101FA/3102FB (per manual) and I am trying to use my zigzag feature for the first time. It is not making a continual zigzag pattern. It will zig and then do stitches that look like long straight stitches. Then do a couple of zigs and then the straight stuff again. I have tried to adjust both the top and bottom tension without much luck. Not sure what I am doing wrong.