does this happen to anyone else?

why is it i can play great one day and win almost all my games then the next day play like a complete bag of crap.

i would understand if i won or lost just a few games in a row but my rating can fly up or down by 100+ in one day...

it kind of makes me feel like learning anything about chess is a waste of time(for me anyway)because it all depends on if mr brain feels like working or not.

is this a problem only i have? can i combat it somehow? is there a secret to bieng a more stable player?or is this just a common thing

i understand how ya feeling, my games fluctuate to, i do find that i play a much better game when not hungry, but times are hard hence i play like crud!

also, i think it could be an age thing? I'm 52 now n dont play half as well as i used to in my younger years, probably hasnt helped you but thought i'd comment anyway

You mean one day you are playing players in a range of say 1500 +-200 and then you get 100 points and you are playing players who are 1600 +-200 and your shocked that the difficulty curve spikes and falls?

Hmm... must be an unsolvable mystery.

We are no machines. When tired, hungry, depressed, distracted, drunk etc. we play worse - a lot worse!

Chess is a game invented by the devil. That's why.

From my own experience when my own play has not been at its highest level , it comes from being to over engrossed with what I was doing and not paying enough attention to what my opponent was doing. 

Im not sure obviously what style of play your used to, but lets assume its a tournament 2 minutes per move. Spend a few more seconds on seeing what the person you are playing is doing. See if you can then incorporate moves that both frustrate your opponent and also advance your own plans. 

One last thing is to slow down. Dont force things. Poor play can lead to frustration and make you impatient.  

Lastly no matter what level you are or what level the person you are playing is they can just stumble into a series of moves that are pure Genius. It happens and you have no way to control that. 

Hope that helps a little. 

Roger