trouble engaging second highest gear etc

[wisdom]

History of the problem is as follows.
Had to get a new rear wheel due to pothole and cracked rim.
previous to new wheel no problems at all.Newish casette and chain and front middle ring.So they can be eliminated.
When I am going down the gears it jumps from 3rd smallest to smallest. No chattering smoothly misses it.
(running 9 speed sora with rapidfire shifters).
I have tried adjusting using the barrel adjuster on the rear mech to no avail.
Also it hangs up when shifting up and down across the range of gears.When I operate the shifter nothing happens then it will drop in of its own accord.Cable seems smooth at mech end.Any ideas please.

 

[wisdom]

Just to add.Chain gets mickled every week currently running with fenwicks stealth as the lube

 

[sittingbull]

A frayed cable inside the shifter body once caused phantom shifting for me. I suspect a gunked up rear mech could do the same.

 

[Citius]

Release the cable from the rear mech, re-set the limit screws, re-attach the cable and re-index...

 

[gbb]

Cassette tightened properly ?
Worth a quick lookee just to eliminate it.

 

[screenman]

When was the last time you checked the rear mech alignment. It should be the first thing to do when setting gears up.

 

[wisdom]

Thanks for all the advice.Ill have a look at all the suggestions and hopefully get to the bottom of it.

 

[Mobytek]

As above, slacked of cable, probs replace it, and start again. Re set all barrels, Largest CR and smalles gear, top jockey directly in line with the smallest sprocket, undo both limit screws, H limit till it hold where you need it to hold in line, pull slack on cable and tighten, shift up through to large sprocket, set limit screw and index up and down.