Consistency

Hello, I'm posting because I'm in a rather frustrating situation these days with my playing. Some pieces I play I can play consistently well. For example, I can play the Waldstein very well. However, there are these two Chopin Etudes, Op. 10 no. 1 and the double thirds etude which I can play well every so often. When this occurs I always feel a certain way. I'm not sure if what I'm feeling is a state of being relaxed but whatever it is I feel in control of my playing. However, at other times I do not feel in control and I can't pinpoint what it is that is different. I'm not aware of certain muscles having different degrees of tension. I'm starting to think it's more of a lifestyle thing. Perhaps it's how much sleep I got or what I ate?! Anyway, has anyone experienced this mystery. Again it's hard for me to pinpoint if I'm doing anything physically different. Thanks for any ideas or suggestions! I know it's tricky to help if you can't see what's going on. Asher