Intermittent no starts can often be attributed to ignition switch failures (the electrical portion).
More often than not, mechanics change the starter because the symptoms lead them to believe the starter is the source of the problem.
In reality what generally occurs is the voltage going to the starter is less than adequate to COMPLETELY turn over the engine. This is not to say that Starter assemblies do not fail because they do, but an intermittant ignition switch problem can be a difficult problem to diagnose.
In most cases when the starter fails it will be the starter solenoid failing noted by a click, click when attempting to start.
Make sure that you check to see if you have 12 volts at the solenoid prior to replacing the starter.
If you do not have 12 volts at the solenoid then look to the ignition switch as the problem area!