hamster wrote:I had the same symptoms years ago on a Shimano mech of similar quality (300LX MTB). Essentially it seemed to corrode inside one of the pivot shafts and this rusted it solid.
Might be where you live, I tend to apply one drop to each of eight pivots on both derailleurs periodically, otherwise its only standing bikes in garden from new that suffer this.
Eye dropper filled with gear oil is best applicator.