intermittent problem - car starter dead, no start

[mlipmd]

I understand how heat changes electrical resistance so maybe explain how the switch would fail, but why does it get hot? It's only on momentarily for a sec. or two 'till the engine starts, but does that arcing heat it up so much it fails? And why would it work again after 1/2 hr. or more of cooling off? When it fails the contacts are apart, no connection, if they were fused together by the heat and arcing it would be constantly on. And the current going through there is only to energize the starter solenoid, not to engage the starter motor itself. This makes no sense to me so somebody enlighten me why this is.