From your description the first thing I would look at is the cable housing ends. If you can see a stray wire or two sticking out - that's it! Remove the ferule from the end, cut the housing nice and square and you'll be good to go.

If your housing isn't square, it will gradually shorten itself during use. When that happens you can adjust the shifting to work right for awhile but, after some period of time, it reverts back to inadequate cable tension.