[phil-l]

In my experience, if the starter is able to crank the engine without problem at least *some* of the time, it's more likely a wiring/control problem - not a problem within the starter itself.

As you noted, 'jumping' the starter directly from the battery is a good way to test this, but can be hard to do depending on how difficult the starter is to get to.

Something that might be easier to test: Can you get a voltmeter lead directly on the starter 12V connection? If it doesn't read full battery voltage while attempting to crank, there's some kind of wiring/control issue.

This test is particularly useful when turning the key *doesn't* crank the engine. If there's full voltage, it's a starter problem. If there's no voltage, the problem is elsewhere.

Of course, now is a good time to check all battery and chassis ground connections.