Quiting as a Winner

[MikeyEdge]

I don't play Hold'em in a casino that often, when I do its either 4/8 or 5/10. Its been a common theme that I will go up over $200, then lose it back and then some. Should I quit while I'm ahead and take the victory, or should I believe in the fact that poker is all one big long game. In theory if you can beat the game, you should be able to continue beating the game, but usually I will loosen up too much and start on a downswing, even though I'm aware to try to not let this happen. Does anyone else have this problem?