Besides worn springs and carriers, there can be other things go wrong. I ruined a left lever by yanking on the cable to try and free a stuck front derailleur. It buggered the ratchet mech. It's a complete sub-assy EC CH 429 along with ratchet EC RE 234 - see here: http://www.campagnolo.com/repository/documenti/en/spares07_A-0307.pdf
and was not economically repairable. I ended up gutting the shifting mech completely, leaving the lever as a brake only, and mounted a downtube shifter for the FD.
So, it's hard to tell.
As to your questions, I'm not sure there's a definite test of the right lever without installing it. I've not heard of one. Some idea of history and wear on it would be useful.
G springs and carriers are relatively cheap and can be installed by anyone who can follow the online instruction videos. You could hope that's the issue with your left lever and try it. And while buying the bits, you may as well buy them for the right shifter too. Bikeman http://www.bikeman.com/store/merchant.mvc?
is a good source. Buy the metal spring carriers, not the "Ultra" ones. Left is 5 EC RE 112, springs 50 EC RE 210, right carrier is 5 EC RE 111, and springs EC RE 209. good luck.
There's sometimes a buggy.
How many drivers does a buggy have?
So let's just say I'm drivin' this buggy...
and if you fix your attitude you can ride along with me.http://www.youtube.com/watch?v=GekiIMh4ZkM