I can't copy to anything to the clipboard

My clipboard just seems to die randomly. I can't nail down when or how. I restart my laptop and it works fine for a while. But later in the day, all of the sudden I can no longer copy anything. I select some text and copy or cmd-C and go somewhere else and paste and it just pastes the last thing I had in the clipboard from earlier in the day.

Also, once I copy something, I can go to the Finder and choose Show Clipboard from the edit menu and it appears blank.

Any ideas?