Originally Posted by cyclezen

That doesn't tell us much on the model/speed.
But if cable housing is moving when you try to actuate, then release/unscrew the cable from the deraiileur (FD or RD ?) pull cable housing a few inches off shifter to expose the cable there, grab cable and see of it slides smoothly - will tell you ff problem lies at shifter or at some point AFTER the shifter - might be an 'partial torn'/unraveled cable stuck at a junction or in cable housing... a rusted/unraveling cable is mostly likely... replace cable and likely best to replace the cable housings as well.
EDIT: ... like Veloz said...