Starter not operating intermittently, still a clicking sound evident.

Customer symptoms were able to be reproduced using a vehicle provided.
Listening to starter operation, could hear slower than ideal cranking speed.
Then on occasion a solenoid click no-crank was experienced.

Ended up tracing this fault to poor quality (under-rated) circuit from battery positive to the starter solenoid relay block.
Excessive voltage-drop on this circuit resulted in compromised starter operation.