Starter or something else?

[Funracer]

My 91Z has been on a battery tender in between drives for probably 10 years. Recently unplugged and went for a 3-4 mile drive and pulled into gas station to refill. After fill up car would not start. Loud click in engine compartment each time i turn the key. Someone jumped me and car started.

Drove home and after i got home car started fine. Plugged in the tender overnight and next day started first try. Drove about three miles then left it parked for 4 hours at the golf course. Would not start - heavy clicking no starter engagement. Got jumped went to Autozone bought a new battery. Started first time. Drove home plugged in the tender and started first try the next day.

Thought the new battery fixed it but problem remains. When on the tender it starts normally. After driving a few miles it won't start.

Starter could be out, but why does it start easily when voltage is topped up and not start (solenoid clicks but no engagement) after a short drive? Alternator?

With holidays and work wont be able to troubleshoot for a week or more. Just wondering if anyone is familiar with this issue.

Thanks