I recently started experiencing shift problems on rear cassette. I cleaned the sticky sports drink gunk off the bottom bracket cable guide. That seemed to fix it for a while. This weekend while in the middle of a long charity ride, it really started acting up by ghost shifting and refusing to stay in gear at the top of the cassette (easy gears where the cable is at high tension). The shifting was fine for the first 40 miles of the ride. Mechanic at rest stop said derailleur hanger looked slightly bent but couldn't adjust it due to lack of alignment tool. He messed with the derailleur stops and cable tension and sent me on my way. A mile up the road, the cable broke off inside the brifter. I rode back to mechanic who was able to fish out the cable end with an ice pick type tool and put a new cable on. I've heard the cable end can become permanently logged in the brifter with this type of cable break so I was lucky it could be removed. Bottom line is to pull the cable and replace it if you start experiencing sudden shifting issues. It's a cheap fix and it could save you big $$ if you have to write off a brifter. Or worse, in my case I would have had to convert to an entirely new shifting system because the 9 speed Ultegra brifter isn't even available anymore.

No, there is no one-size fits all for a particular symptom. Yes, cable problems are often the cause of irregular shifting, but lubrication is in many cases sufficient.