[Yawkey]

Most of the time this happens to me due to over confidence. I would suggest going ahead and just reviewing some of the basics about the particular games you are playing in, that can help settle your play down when you correct some of your bad plays (this works for me anyway).

The other posibility (more so in live games as opposed to online), is that your opponents are getting a good feel on how you play. You may have to mix it up some and say ok I've been playing tight for the past hour, now I'll loosen up for twenty minutes and then tighten up again when they start catching on.