shifting problems

This is caused by one of two problems: insufficient cable tension or set screw adjustment. Cables stretch over time, which will affect shifting. Many, if not all systems, have tension adjusters in line from the shifter to the derailleur. It is basically a hollow knob at the end of the cable sheath that the cable passes through. Shift to the big ring (as far as it will let you go) and turn the knob until the derailleur lines up properly and allows a clean shift. Make sure it will then shift back to the small ring.

There are two set screws on the front derailleur. One sets the outer travel limit, the other the inner travel limit. Once you have the cable tension right, you can play with these to ensure you have enough travel to shift, yet not so much that you will throw the chain off either ring.

This takes time and practice, it can be tricky. It's easiest to do while on a stand or in the trainer. If you like to play with things and know which end of a screwdriver to use, have at it. Worse case is you put it even further out of adjustment and take it to your LBS.

I'm one of those people that have to know how things work and then do it myself. If you are the same way, pick up Zinn and the Art of Road Bike Maintenance, by Lennard Zinn.