Scott,
I've been living with this problem on "The Evil Twin" since I bought it. I didn't think it was going to start, a few times, on my drive home from New York. I just have to keep turning the key, over and over, until it starts, but, it eventually does. I've checked the starter, no problems there (snugged nuts, replaced grounding wire connector). I've changed the battery terminals, problem persists. I've tried electronics cleaner in the ignition switch, still no help.
The first time I saw this problem was at Reading III. A fellow had replaced three starters and still, no go.
Try the suggestions that Herb outlined, and, if it doesn't help, I'm guessing it's in the ignition switch. Drop the underside cowling on the steering column and check for bare or loose wiring. That's gonna be my next check . . . one of these days.
Oh, and sometimes, if I turn the headlights off when the problem surfaces, it starts right up. It's a quandry.