I've had the intermittent start issue on 3 different cars over the last 30 years:
1. Baked direct-drive starter on Hemi with headers; might click & not spin starter, might click & turn over.
Adding heat shield on rebuild of starter fixed that, then I got a Robb MC starter and really fixed it.
2. Faulty ignition switch, same symptoms as above
Replaced switch.
3. Loose post on starter relay, either no click or would start; wiggle post = start.
Replaced relay. (4-speed car, no NSS to confuse matters)