In the past few months when searching keyword terms for my website/industry we have come up on the first page for many terms often only to be no where to be found two days later and then back on top 24 hours after that.
I can't remember the exact source right now, but there was a comment by Matt Cutts about results not clicked on being less likely to show to the same visitor on subsequent searches [or on deeper pages of the same search]
or something along those lines, and those comments make me wonder if that mechanism could be the cause of some of the randomness some of us have seen, because I've seen it myself.
When the preceding is combined with:
When you do search for your keywords, do you clear your cookies first? This could be one of reasons why you see a different result on browser you are normally using versus browser you do not use or use rarely.
It makes quite a bit of sense to me to think there's something algorithmically [technically heuristically]
changing the results based on the previous clicks of a user [or browser]
so I think cookies, logged-in v. logged-out, and even browser sniffing could definitely have an impact on what's seen in the results by any given user [or browser]
There is also possibility that your queries could be hitting a different data centre which could result on different SERPs you are seeing.
This is also a definite possibility, in-my-opinion, but if it's been going on for months I'm not sure if "datacenter alignment issues" are necessarily the most likely cause, but at the same time, they certainly could be.