Just tell the customer as it is. You are not here running a charity, you will look for the fault and charge the hourly rate, if you do not find it this is because there are more tests required or the faulty item tested ok during the test (it is intermittant). If you suspect a part you are diagnosing it to the best of your ability and can not guarantee this part to cure the problem, there may be other faults. If customer agrees - do the job. If customer does not agree - send job elsewhere.
Got to be tough with them.

Graham