i've been having this exact same issue! (mine is an X5 though)
i discovered the mech was actually on wrong. It's the little tab that the B-tension screw nestles against that was rotated round the wrong way. So I'm hoping that fixing that will cure it.
Take the chain off, put it in top (lowest tension) and hold the cage forward in the position it would be in when the chain is on. Now push the mech body towards the bigger sprocket, as it would move when changing, then let go. Does it spring back?
I tried altering the B-tension screw, but then i noticed that it was sort of not having any effect because it wasn't nestling against anything, I thought hang on a minute, this is wrong, so I took it off and put it back on with that little cam-shaped gubbins the other way round, and the mech now sits lower than it was – it was almost high up enough to foul the frame before.
Now when i do the above test again, it passes – it springs back much more readily. Haven't trail-tested it but hopefully that will cure it.
maybe getting it oriented wrong is only possible with sram mechs?