Can you help me solve screen mystery: "mis ing let ers?"

This week my iMac has had two bouts of weird screen behavior. The letters in the menu bar start to go missing. (Example: Fi ef x Fi Ed Vie His ory Bo k mar s) Then, letters all over the desktop start to go missing. (Folder titles start to look like this: Sch ol Pr jec ) When I reboot the problem clears up, but it has happened twice in one week, so I'm wondering if this is going to get worse. Do I need to delete a preference or do some memory cleaning??