Bicycle Mechanics - Shimano 105 front shifter issue

I just bought a used Giant TCR 2 for my wife and during the tune up, ran into an issue (well, ive run into several) with the front shifting system. Its a double chainring, but when I shift to the large ring, the shifter pulls it up the the ring, but wont hold it. Anyone know why this might be?

Most likely its one of two causes:

1. The outer limit screw is too tight and won't let the derailleur cage move far enough
2. the cable is too slack and the shifter doesn't move the derailleur far enough.

Try to move the derailleur by hand while turning the crank. If it moves outward enough to shift to the big ring, the cable is too slack. If you can't move it far enough, the limit screw is to tight.

thank you for the response, but I realized that the way I wrote the first post does not clearly describe the issue. The derailer is working properly, but the shifter, once shifting up to the large gear, cannot hold the gear.

Imagine the bike has no chain. If I shift to the large gear, the derailer goes into position, but the shifter cant keep it there, so as soon as my hand releases the shifter, the derailer comes back down.

I hope I am making sense.

Does it click when you push the lever all the way too the left?

If not, perhaps broken shifter - I've never encountered this problem before, but I think someone else on here a few weeks ago had the same problem. Apparently 105s can break if you force it into the large ring and his/her issue was resolved by replacement.

Here it is; might be a similar issue perhaps:
http://www.bikeforums.net/showthread.php?t=445502&highlight=

If the shifter is working, i.e., it will click into the large ring position, then you have cable tension problems. The cable is too slack and will let the fd retract too much when you release the shift lever.