Pretty much all have been covered here but I would break it down this way:
-Start with getting a set up by a qualified guitar tech. This will include intonation with the bridge saddles and when you do the set up, mention the twang etc and ask to have the nut looked at for a possible bit of filing (remember, filing is how the slots are made in the first place). Also, replace the strings...start fresh.
...if that doesn't work...
-Look at the tuners. While's it's probably not the tuners, Klusen's that come on many Gibsons are not great tuners. They're ok and my ES has them and they work fine, but this could possibly be a culprit. If I did replace my tuners (which I won't because they work fine), I prefer Grovers.
...if that doesn't work...
It's the atmosphere your guitar exists within (heat, cold, humidity). For lack of a better way to explain it, I ended up having my ES willed to me. It was my wife's uncle's guitar. His old house was always damp or humid and just was not a good temperature controlled house. He told me that he just could never play it and asked if I would take it for 'permanent storage'. So I did and when he passed, the family willed it to me.....and you know what? I almost never have to tune my ES. So, temp can be a factor but it is usually in extreme circumstances.
So, that's my breakdown....go with the most likely to the least likely.
