I've seen this happen on my guitars and it always has to do with temp and humidity. Make sure the place you are storing the guitar isn't being exposed to temperatures that are on the extremer such as near windows, in direct sunlight, near vents, etc. Also monitor what humidity the guitar is in. If the guitar undergoes abrupt humidity change when putting it in and out of the case, you'll also notice the guitar going out of tune. Try putting the guitar in a place where you know the temperature/humidity will remain stable and mild and your problem will probably go away.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening