the clicking noise is coming from the starter solinoid that is trying to engage the flywheel. the clicking noise can be due to one of two things.
the first is a low battery. If the battery is discharged it won't have the electrical energy to engage and turn the starter. This is the most probable cause of your problem. Batteries do generate eletricity through a chemical process so that they will recharge themselves to a certain extent and that could explain th e'sometimes it will start". However, the usualy fix for this problem is to replace the battery and to check the charging system to see if it ran down because of age or because of a problem with recharging the battery.
the second possibility is the starter solenoid. that would require the starter be replaced that is much less common than the bad battery.
before you do either, check the connections on the battery for corrosion and looseness and make sure the connections on the starter are tight. This is again, much less common than a dead battery but loose connections cause high resistance which will cause the starter to not engage.
hope that helps
