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RJ lost consistency because he started having a lot of back problems...injury is a common vector. Same thing happened to Batista. This has robbed him off the drive in his back leg so he doesn't get the snap on his slider (it's more like a hard slurve now) and he doesn't throw 97...he throws 91 (and that IS a huge difference).
KGJ lost power because his knees went to hell. That tends to be where a lot power comes from afterall.
But some guys lose consistency just from fatigue...not from obvious injuries. Lots of pitchers especially just start becoming increasingly wild and giving up more mistakes and getting kncoked around and never get it going again.