Sounds like your cable housing is contaminated. Been doing any riding in wet conditions? How about dirty conditions?

To start, remove the cable loose from the derailleur and shift with one hand and hold the loose cable in the other. Is the motion smooth or rough? Try removing the cable completly and checking for any grit or grime (usually more at either end, but even more so at the derailleur.). Use some wet lube like Tri-Tech or Finish Line Cross Country and drip some into the housing. Wipe off the cable and then insert it. Run it through, move it back and forth and the remove it, wipe it down again, drip more oil, ....repeat until cable moves smoothly. If you cannot get it smooth, replace both cable and housing!

Replacing your cables will make significant improvements for not a lot of $$$$$. Doing this on a yearly basis wouldn't be a bad idea if you ride a lot in bad conditions!

Good Luck
L8R