and the most common problem of all is a hi-resistance ground circuit.
The ground is from battery to block to bellhousing to starter body.
I have seen many cases where the starter nose oxidizes to the point of providing a poor connection, and then, when you turn the key, you never know what you are gonna get.
The test is simple; just grab your jumper cable and jumper from the starter body to the battery negative, then turn the key.