I had this problem with my phone but I think it was flexing the phone with the keyboard open and my thumbs on the keys brought it back, issue came back once more and the same fix worked. It is a weak solder joint and I suppose the best fix is some time in a reflow oven, has anyone tried this? Are there added components that would make this a problem?
http://www.instructables.com/id/Toas...Soldering-BGA/
I used this toaster oven reball method several times to fix pocket flex and drop damage to the CF wifi card back when I had an old Linux Zaurus.