i have one too and it has been really good actually.
this may sound a little weird but do are stretch your strings after you put them on?
what kind of strings are you using?
the next time you do a restring, take a close look at your fine tuning screws and make sure that they all move smoothly over their full range of motion. if you get one that binds up at some point this could be causing your type of problem. use a little vaseline to lubricate them, but don't go over board. you can also lubricate where the tip of the fine tuners come into contact with the string lock screw (the one's that hold the saddles).
when you string up, make sure your fine tuning screws are in maybe a turn past center (giving you more space to tune lower). after you tune up and stretch your strings, play it a bit and make sure everything is relatively stable before you lock down the nut. now when you lock down your nut the tuning will go higher, so back off your fine tuners until you're in tune. the idea is to have the fine tuners as close to center position as you can get them. i've found that on any locking trem, tuning stability tends to drop off as you approach either end of the range of the fine tuners.
forgive me if you know all this. i'm not trying to be an ass or anything, i just don't know your experience level.