Could be a whole variety of things... are you're strings properly stretched (they're only a couple of weeks old)? Or perhaps the trem springs are on the way out?
Do you play often or does it get only occasional use? Sometimes if you haven't played for a while the springs can develop a "memory" and when you go to use the trem all sorts of oddities can occur with the tuning. I had this problem myself a while back. The answer is to play more... use plenty whammy!
Apart from that, slipping nut pads/loose nut pads, loose nut, worn knife edges, worn posts, loose posts, strings not locked down properly in the bridge saddles, incorrect setup... double-locking systems are a minefield!
Have a look at the guitar and check the obvious things - neck, nut and other screws tightened up, everything in place, nut pads locked down properly, strings secured in the saddles, strings fully stretched using the proper technique (see Ibanez Rules). If all is OK then you may simply have worn knife edges. It's not that daunting to fix if that's what it is... although it is a last resort.
Hope this helps fix your woes!