I don't know about the specifics of your front derailleur, but your description sounds like it might be a mis-routed cable. Some derailleurs need the cable to run over a tab - usually a tab built into the washer under the cable fixing bolt - to route the cable further from the pivot point. If the cable runs under this tab, the lever arm is too short and the derailleur will move too far with each click, and the tension in the cable will be excessive, which might also affect the functioning of the shifter.
As for the possibility that the shift lever is faulty, if it pulls cable and clicks into place and releases cable with a click when the smaller lever is pushed then it is almost definitely not the shifter. Faulty Shimano shifters usually don't give a click in one direction or the other. Of course there is nothing stopping you from flushing the shifter innards with WD40 or a degreaser, I doubt it will make much difference.