I'm having a problem with shifting between chainrings, i suspect its the shifter. I can't seem to get up to the largest chainring. When I press the button, the derailleur moves and the chain goes up, but as soon as I let go, the derailleur jumps back down to the middle chainring taking the chain with it. It seems like the shifter isn't "locking" into place. Any ideas?

I'm going to reply to your post on the commuting forum as well, but...

It's entirely possible your shifter and derailleur are fine. Your front shifter is designed to allow you to pull slightly more cable in with it than it will hold. Essentially, if the system is working right, you pull the shifter past its "catch" point and it pulls some extra cable, (helping make sure the chain is forced to jump to another chainring) and then it slides back to its catch point, letting a little bit of cable out. When you have too much cable, pulling the shifter past its catch point pulls the derailleur over enough to shift gears, but then when the cable is let out while the shifter goes back TO its catch point, the gear shifts back.

The chance is also quite good that your shifter isn't working. I don't know what kind of shifter you have. (is it a grip-shift where you twist the grip, is it a trigger-shifter where you have two buttons, one to upshift and one to downshift, or is it something else?) But shifters typically can't be repaired and are cheap to replace. But a shifter that isn't working correctly can sometimes be convinced to do its job by a liberal dose of WD-40. If the thing's not working right, you don't have much to lose- just spray some of the stuff on the shifter's innards and see what happens.

I tend to only use WD-40 to clean the crud off of stuff. For a light lubricant, I use silicone spray.

Did it ever work right?

If your answer is "Yes," almost surely you just need a touch more cable tension. Find the barrel adjuster for your front derailleur and turn it counterclockwise 1/4 to 1/2 turn and that should fix it.

If your answer is "No," I'd recommend looking up front derailleur adjustment on the Park Tool website. disconnect your shift cable and start at the very beginning with the position of the derailleur on the seat tube. Don't skip any of the steps.

Thanks everyone for your help. A nice dose of lubricant and a little more cable tension fixed it right up. I really appreciate the advice.