I replace all cables once a year as part of basic maintenance but sometimes it's not enough. A cable frayed between the bb and front deraileur and it started shifting like s@@t. Thought it was the deraileur till I got home and gave it a good look. Cable was only a few months old...

Back the cable all the way out and away from the bb and check it for damage like broken strands. Check the plastic bb guide for damage (crushed or packed with crap?) Hold the end, and click the shifter, do you feel it pull? Does it stay pulled or does it release what it pulled when you finish the shift?