Adjust a Shimano 105 9x rear derailleur Please help me (re-)adjust my .Shimano 105 9x rear derailleur of my ancient Shimano 105 3x9 gruppo.  It's a RD-5501, as my research seems to support. 

The prequel: After working fine for many kilometers, the pulley cable came loose. I re-installed the cable, of course. 

But ever since, the rear derailleur seems to shift randomly into a neighbouring gear without me moving the shift levers. It seems to happen mostly when the chain is on the "middle" cogs; when I shift into the largest or smallest gear, the chain stays on its cog just fine. 

The last procedure I tried was as follows:


Loosen cable
Chain on greatest sprocket in front, smallest cog in the rear; 
2.1. turn the crank and loosen "high" screw until the chain starts rattling
2.2. tighten the "high" screw until the rattling stops
Turn cable tensioner to most-"loose" position
Install pulley cable
Chain on middle (!) sprocket in front, largest cog in the rear;
5.1. tighten "low" screw until rattling starts
5.2. loosen "low" screw until rattling stops
Try out all other cogs - all gears seem to shift nicely.


Still, when I ride only a short distance, once the chain is on a middle-ish cog, 4th to 6th, it jumps to the next cog intermittantly.

What am I doing wrong?
I think the problem with your procedure is that you are adjusting the limits but not adjusting the indexing explicitly. Try working through the detailed procedure that Park Tool recommends. There is both a video and written instructions on this page.

Check your derailleur hanger is straight. A special tool is required to do this accurately, but you can eyeball it by picking up the back of the bike and looking down the chain. The derailleur cage should look like it is parallel with the chain rings.

Another possibility is that your derailleur is getting worn and has developed some slop. There may be play in the parallelogram pivots, or the top jockey wheel has excessive play (although note that top jockey wheels normally have a few mm of lateral movement).