One trick is to leave the test wire hooked up and you can even run a longer wire back to the cab and leave a volt meter connected. That way when it starts normally, observe the voltage. Then when it does not start, check again and if the voltage to the solenoid has dropped, there is an issue upstream of the starter. If the voltage is OK, there is an issue inside the solenoid. And if that is the case, don't hesitate to open up the solenoid for a look-see. Not uncommon for a newly "rebuilt" starter to have worn contacts. Why? Since it tends to be an intermittent issue, if the starter tests out fine on the bench, many places will just clean it up and put a fresh coat of paint on it and call it "rebuilt".