I know this has been turning itself off over the years with not a hint at why. But recently it was way worse on my machine - it would go off every minute. I finally figured out that it was my SpacePilot that was causing it! Every time I manipulated the model with my controller, it would uncheck the setting.
I restarted SW - no change. I rebooted the machine - no change. I updated the 3DConnexion driver to 3.6.7 - that did it.
I am on XP64 SW2008 SP4.0.
WT