Don't you just love SEO sometimes Robert?!
It is very frustrating when you think you have done everything right, but don't see the results you are expecting. Obviously, I don't know the full details of the keyword and the page you are trying to rank it on, but I don't think I have ever had what you have described having done what you have described.
Recently I have seen some odd bouncing around in SERP rankings, nothing very dramatic but bigger bounces then I would expect and negative ones at that when I would have expected positives. My only thought is I wonder, with the shake up in search by Google and the growing move to semantic search and away from a boolean keyword-based algorithm, whether this is the cause of such volatility of which we can expect to see more.
You mention 'keyword'. I know 'keyword' tends to be synonymous with what is more accurately a keyword phrase, but is the keyword you are trying to rank for just one word or a multi-word phrase? If one word, then I would expect to see a big swing in ranking results because in many/most cases how can you draw a semantic understanding of the searcher's intent through one word. I don't think it's linear, and although this is a generalisation, I think the longer your keyword phrase the less volatility.
I don't think the above is really an answer for you, but I hope it helps maybe in some way.
Peter