I have talked about this with a couple of my teachers, in the past, as well as a number of top level pro players and the concensus has been that bad days are just part of playing - the body cannot keep up the extremely high levels of concentration and performance all the time. The solution is to work hard enough that your bad days are better than everybody else's good days
I have heard some world class players give performances on their bad days, I have known that they were having a bad day purely because I know how they usually sound and the fact that I knew the music that they were playing - to the general public in the audience, they were still awesome.
I have yet to come across anybody who says that they don't get some days that are better than others.