Is it related to a time-of-day query? I see what I can only describe as QDF fluctuations sometimes.
We have run the same keyword rankings every hour for 48 hours before. All 48 were unique sets of results ... the top "100" ended up including almost 300 different URLs. A lot of the fluctuation was on page 1. We saw that a search that "deserves freshness" ended up fluctuating MUCH more. Say "restaurants near Melbourne." In the morning we often saw different results to dinner-time.Over a couple days it's hard to say but especially on mobile, those things are changing fast.
(It's the old context question. If you type "who is the President" you get info about Obama. If you type "who is his wife" you get random info. But if you say those two questions in Google mobile, it returns Michelle Obama info.)
I do think your exact issue may be some testing, though. We have an idea that the newest/last release Penguin is coming soon. Testing makes just as much sense in your case. (The results often shift, go back, shift, go back.) I would start checking links & cleaning up in any case.