mtb deraileur for 1998 s-works (8 speed) I recently installed a Sunrace R90. The chain jumps in the middle gears. This may hurt my sprockets. I wonder if it is the shifter being worn that causes the jumping back and forth, for example from 5th to 6th. (The shifter is Deore XT.) Also one screw or spot on the hanger is stripped. The bike mechanic says he would start by changing the housing and cable first. I have already had adjustments made.

Any thoughts?
If the derailleur is correctly adjusted at one spot in the gear range, but not throughout the full range of movement, this can indicate the gear hanger is no longer aligned properly.


If the hanger is bent towards the wheel, each step in the derailleur's movement will be too wide and its path will be shallower (it may hold the upper jockey wheel closer to the large sprockets than it should, or if the B-screw has been adjusted to keep it away from the large sprockets, the jockey wheel would be further away from the small spockets than it should. 
If the hanger is bent away from the wheel (only really happens when someone "overcorrects" a bend towards the wheel, or an impact occurs when the rear wheel is not present), the derailleur won't move far enough for each step and the path will be steeper (closer to small sprockets but far from large sprockets).


If you get correct shifting at the low and high gears but not in the middle, your derailleur is probably configured correctly and your gear hanger undamaged, but your cassette is probably worn out in the gears you use most often. You will experience more problems with new chains on old sprockets.

A common rule of thumb suggests the service life of a cassette is about 3 chains, assuming you don't delay replacing worn chains and make reasonably even use of each sprocket.

Chain wear gauges are readily available, costing about the same as a cheap chain. They often look like a capital I (or very wide H, depending on your point of view). Usually, one side is calibrated for +0.5%, the other +0.75%. On each side, one "finger" will be hooked, the other straight. On a tensioned chain, place the hooked end into one of the chain gaps. If the straight finger drops into another gap easily, the chain has worn such that each pin is that percentage further from its neighbours than spec.


+0.75%: Replace the chain immediately
+0.5%: Replace soon.


The teeth of the sprockets, chainrings and jockeywheels are worn more quickly when the distance between chain pins is over spec, so for a cassette's "3-chain" life, it's better to change the first chain early, whereas the third chain can be run slightly longer since the rest of the drivetrain is approaching end-of-life.