I'm seeing it in position 1. Considering this happens often in the weekends, which is always the time that things change in Google, (probably because they update certain things during weekends usually), I'd wait a bit until things have settled down.

Sudden drops can also be caused by age factors, but that usually means you´re out for pretty much 95% of your keywords. The fact that this happens for just 1 keyword, is an indication of something else and it is the better to wait and let the storm pass first.

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