My main keyword ranks. Then disappears. Then ranks. Then disappears

I'm a local business with a pretty decent DR and backlink profile and my main keyword on the homepage just isn't being picked up consistently by google. It's a super competitive keyword and when I searched for it just now I was at the top of the third page (and was like 'woooo!'). Ran another search and I'm gone. Have run five more searches since and I"m gone. It's super frustrating. Anyone have any insight on this? I feel like my page is well designed and has enough quality copy with the keyword mentioned.