Unstable keyword

Hi

For the past year, we've been targeting a keyword which is highly relevant for our business, but the placement has been very unstable. See attached images.

We've been consistently ranking as #1, but every so often the ranking is completely lost. This has been going on for 1+ year. It's not a competing article on the keyword, we've tried changing both image tags and changing the keyword density but nothing has made a difference yet.

Any ideas what might cause this? I was crossing my fingers that it would eventually settle and start ranking, but the issue is persistant. Please help

https://imgur.com/a/XmaLnMs

If you can get some high quality outside links that are relevant to the keyword.

What you have there is a Google Dance. You need to improve the authority of your website.