The most common culrit to a guitar tuning itself UP by itself are changes in atmosphere (temperature and humidity). I think the one post up above explains this. As you look at a guitar in terms of basic materials, all those materials react differently to changes in temerature and humidity. Metals stretch and contract much more rapidly under temperature changes than wood does... Wood reacts much more rapidly to changes in humidity. It's important to regulate these things as much as possible to some sort of consistency. If you're into aged guitars, humidity will speed this process. Often you'll see checking and cracking in the finish where the wood swells and contracts during increases and decreases in humidty. It's particularly bad for guitars like Les Pauls with neck joints that are glued. Over time this will weaken the joint. More than likely this is your issue...
Strings expand with heat and contract as they cool. Basic metallurgy. A string that contracts goes sharp and a string that expands goes flat...
Also, binding at the nut and at the bridge can cause a guitar to suddenly seem sharp a bit while playing. Try some simple graphite lubricant at the nut when you change strings and at the bridge... This will help your guitar to return to its natural state after big bends and lengthy playing periods.
