I think I'm 99% sure what's the problem with the buttons. I took apart my camera yesterday and at first I didn't really pay attention, but now it dawned on me! The problem is the battery. The battery bubbled up a lot!
The battery is very tightly sealed in some plastic/aluminum tape. Well guess what, the battery was really INFLATED. In my case I'm sure it's because of extreme hot temperature. So instead of being flat, it became almost round...
So there you go...The battery expands (rather air/gas inside the enclosure) and it pushes against the board of the camera, which in turns pushes against the metal case...
So when it cools down, it contracts somewhat and they work just fine once again.