Clipboard problem

Every now and again the clipboard becomes unavailable (NT4, SP5). Opening the clipboard viewer can show the hourglass or a flashing cursor/hourglass. Rebooting usually “cures” the problem, but not always. I created a clipbook file, and that seemed to clear the problem on a machine that had been rebooted several times for this problem, but I’m interested in what is causing the problem. Any ideas/suggestions?