Rear derailleur and front shifter are not indexing "together"?
If you need to replace the cable, try this:

Shimano 105 Shifter Cable Removal


Take the bike for a short ride and shift into the gear that gives the shifter cable maximum slack. For the front gear shifter, maximum cable slack corresponds with the lowest gear. For the rear gear shifter, maximum cable slack equals the largest gear.
Loosen the bolt on the derailleur that secures the shifter cable with a 5-mm hex wrench. The shifter cable is attached at one end to the Shimano 105 shifter and at the other end to the derailleur.
Cut the metal cap, called the "ferrule," away from the end of the cable, using cable cutters. Grab the cable where it enters the derailleur and pull it free.
Squeeze the shift lever on the Shimano 105 gear shifter to expose the head of the cable. The cable head is located on the side of the top of the shift lever.
Grab a portion of the cable and push it forward, forcing the head of the cable out of the Shimano 105 gear shifter. Grab the head of the cable and pull the cable free.


Shimano 105 Shifter Cable Installation


Thread the replacement shifter cable through the hole in the Shimano
105 gear shifter. This is the same hole from which the old cable
emerged. The cable has two ends, with a metal fitting at one end,
which is designed to anchor the cable inside the shifter.
Pull the shifter cable taught until the metal fitting is fully
anchored.
Pass the cable along the bicycle. The cable follows the exact route
of the old shifter cable, passing through the cable housing and
cable stops before arriving at the derailleur.
Thread the shifter cable beneath the derailleur bolt and pull the
cable tight.
Tighten the derailleur bolt onto the cable, using the 5-mm hex
wrench. The cable will be excessively long and will need to be cut.
Cut the cable just past the derailleur bolt, using the cable
cutters. Leave enough cable, approximately 2 to 3 inches, to allow
for future adjustments of the cable.
Press a new ferrule onto the tip of the cable. Squeeze the ferrule
onto the cable using pliers.


from: http://www.ehow.com/how_7706040_replace-shifter-cable-shimano-105.html