328 starting enigma

Recent occasional phenomenon has occurred when going to start the car: turn the key, no starter attempt or any normal effort. Has happened after sitting anywhere from a day to a few days, as well as an hour after driving (i.e. driving long enough to charge a battery enough to at least get a crank). Batt seems to be delivering a healthy output (lights, other power-consuming functions work fine w.no hint of draw down), and when this last occurred my charger indicated 90% charge.

In one of the last 3 cases I jumped it and she started fine first try. In another, I simply switched the key off/on a few times and tried again and she started normally. In the last, I hooked up the Schumacher charger (which does NOT have a jump feature and indicated 90% charge) and she immediately started. There's no way a 10 second charge could have taken a battery too weak for even turnover to normal starting, and is further corroboration to me that the battery is not the issue. The terminals are solidly connected.

I do hear relays clicking or some sound when I was getting no starter response. All other instrument lights, etc, were on as normal. And yes, the clutch was in, gear N. Could there be a starter issue? Maybe a connection on the starter?

Thx.