My starter just clicks every now and again.
When the jackshaft and ring gear are perfectly aligned it takes very little amperage to engage the starter. When they aren't aligned perfectly it can take up to 40 amps to compress the 3 springs (jackshaft, clutch, solenoid) so the jackshaft gear will walk itself into the ring gear.
Take a small piece of wire (10 -12 gauge), pull the "S" terminal wire out of the starter and jump between the battery hook up on the starter and the "S" terminal. This eliminates everything upstream. If the starter engages fine the issue is upstream, meaning that the starter is not getting a full 12 volts to the solenoid.
The gauge of wire from the relay to the "S" terminal is too small (we recommend 12 ga)
Another cause of clicking is the jackshaft bolt is too tight. It can cause the jackshaft to bind. In this case, slightly looser is better. The lock tab is enough to keep the bolt in place.