Like you Christie, I am a professional speaker / presenter and have been having the same problem when using a Power Point and a projector. I've not used Keynote for a presentation. I use my charger when presenting so I'm not running off the battery.
I've gone to my System Preferences > Energy Saver and both the Computer Sleep and Display Sleep to "Never" when presenting. It didn't fix the problem for me.
I unchecked "Put hard drive to sleep when possible." No difference.
The unfortunate only "solution" has been for me to close my Mac's top and reopen it - not a full shut down (thankfully). This takes it out of PowerPoint presentation mode. Once I open the lip, I then put it back in the PP presentation mode and continue until the next incident. It is embarassingly unprofessional to have these tech glitches, resets, and waste of people's time.
I've done these type of presentation with a mac and projector for 15+ years. This has just happened in the last few months. My OS is currently 10.8.2.
Any ideas out there would be most appreciated. I'm presenting again on March 16.