Elna Carina Zig'Zag Stitch Issue

Question

Hello,

I own an Elna Carina sewing machine that is about 25 years old. It has always worked fine and I had it cleaned and checked out for the first time about four years ago. The technician said it barely needed servicing because I had maintained it so well. I have not used it all that much since then. The problem I am experiencing is with the zig-zag stitch. If I stitch one stitch at a time it works fine with the properly set length and width. However, when I begin to stitch faster, the zig-zag almost disappears into a striaght stitch. This is a first for me. Any suggestions???