Zig-Zag sewing machine problem

[moving on]

Hi there everyone, I'm new to this forum having discovered it looking for help.
My sewing machine is refusing to zig-zag evenly, it's fine for a few stiches and then straight stictches a bit, and then back to zig-zag.
I've tried rethreading, checked upper and lower tension, changed the needle.
Has anyone any ideas ? I live outside the UK and have never knowingly seen a sewing machine shop here. Moving On