On yesterday's ride, I got to talking about changing my cables and housing once a year. I normally do it in October, but I had a sense I needed to do it sooner; I've had problems with shifting onto the big ring and with spontaneous downshifts. So I changed the brake and derailleur cables and housing today after Ken G's ride today, and this is what I found:
That's a cut end of the old derailleur cable. Protruding from the cut end, you can see the longitudinal wires that are molded into the housing that make indexed shifting possible. They have worked their way out of the housing to a distance of about 7mm, more than a quarter inch.
THAT's why you need to change your cables and housing every now and again.