My shifters suck

[moderncyclista]

I'm thinking cables/housing. Two years of mountain biking...makes sense this would be the issue. I would wager on the 8"-10" of housing right by the rear deraileur as the culprit.
+1 Melalvai on the kinked cable too.

+1 on this and the other suggestion by Melavai!

My Jamis had this very problem. I thought it was the STI shifters but it just needed new cables. Whoever had originally installed them, sucked at it. (I had picked that bike up cheap off eBay.)

So, check the cables out first. It wasn't something I noticed with my naked-eye. Let the pros have a look at it.