It is disconcerting that players that normally would never win against me are able to beat me on my bad days. My playing strength varies a great deal. I think it has to do with stamina and physical fitness. Chess is extremely demanding intellectually and even physically due to the great strain of making fast and accurate calculations in games that often permit less than twenty minutes per side. On certain days, I see many moves quickly, but on bad days, my thought processes slow down, with devastating impact to performance. I would characterize my strength as erratic.
I would like to be more consistent, of course, but there are many things one would like to happen. . . The best I can do is to manage, avoiding chess on my slow days and only playing when I feel strong.