How to stabilize a keywords position that keeps dancing?

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Anyone has experienced your kw's doing this? ^

The keywords are (example, not actually toaster) mini toaster, tiny toaster… "mini" has about 50% more search volume so I targeted that in URL, site title, H1, etc. It's the red line.

But I also want to rank for the "tiny" version which is going up to similar position to mini toaster then jumping way back.

I wonder if google isn't sure if it's relevant, so I changed some headings from just…

How to stabilize a keywords position that keeps dancing?