You might check to see when you have your screen saver set to start.
Then look at energy saver prefs and see when you have Sleep and Display Sleep (if set) set to start. You could also check energy saver options and, if you don't need them, uncheck them.
You could be having a problem is any of these are set for the same time. I have the Energy saver settings set to kick in after 1 hr of no use and the screen saver set to 15 min.