alanturing

I picked up a second hand mountain bike with circa 2003 Deore 9 speed components and rapidfire shifters.

I've gotten the rear derailleur to work perfectly, but I'm having a heck of a time with the front.
I'll adjust it on the stand, and it will shift well for a little while when riding, but then I start missing shifts. That is, the shifter might move all the way in with no resistance and without catching, or it's really hard to press and shift. Once this happens, front shifting becomes amost impossible. I replaced the cable, and that didn't work. I even swapped the shifter with one I had on my old bike from around the same era (maybe a year or two older) and it does exactly the same thing..

I'm thinking of replacing the shifter and the derailleur, but I was wondering if anyone else had some other ideas.

Torchy McFlux

Unbolt the cable from the front derailleur and move it by hand through its range to make sure it's not binding. Soak the pivots with penetrating oil if there's any stickiness.
Spray the internals of the front shifter with WD40 and let it soak in. Sometimes the little pawls and springs in there get dry/rusty/sticky and need something to free them up. Test the shifter by gripping and pulling the installed cable with one hand while operating the shifter through its range with the other.
Make sure the cable housing is clean, kink-free, and has proper ferrules capping the ends. Make sure the cable barrel is installed in the shifter properly too.

LVRider

Another vote for spray the internals of the shifter with WD40, Boeshield or Superlube, just stick the tube in the hole in the back of the shifter and spray away, moving the lever at the same time. You'll have to re-do this from time to time.
I found this out last year when I spent more having my shifter rebuilt than I could have bought a new pair for.