"If it was the starter then it wouldn't have started at all?"
is not correct. That is exactly what is meant by an intermittent problem. The potential worn contacts are inside the solenoid which is part of the starter motor. It can be replaced separately, but the time to do that, and hope it was done right, is usually not as good a value as simply replacing the entire starter with a warranty.
Before we even go that far to assume the problem is caused by the starter, lets wait for it to act up more often. Intermittent problems show up more and more often as they get worse. If it becomes a permanent problem, THEN we'll be able to do some tests to determine what is causing it.
Also, don't overlook the battery. If it is original, it has exceeded its expected life span of five years. Typically they fail suddenly and cause a no or slow-crank condition, but it is also possible for them to be just weak enough to not be able to engage the starter solenoid every time.
Friday, January 2nd, 2015 AT 12:18 AM