I just want to say I came to the same conclusion as you (plaing 3-min games at chess.com several times a day, being in the 1250-1300 range) - quite a striking difference between good days and bad days, and only partly related to what I would expect (because of sleep deprivation etc.)
Also clearly better results in the evening than in the morning.
I have been thinking many times about statistically processing the results, but don't really have the time :)