Won't start shortly after running for 20-30 minutes or so

[FeeAught]

True novice here. I'm having a problem with my 79 spider. It runs great, but every time I drive it for 20+ minutes or so, it won't start if I try to fire it up shortly after. I can jump it and it starts up without issue. The battery is essentially a brand new optima, and when I'm jumping it, it has plenty of juice. If it sits all day or all night, it starts up no problem. Well, it might click on the first few attempts at turning the ignition, but if it clicks, I know it will start eventually. When I don't get a click is when I know it will not start. It does not happen at all if the car had been at rest for an extended period of time (several hours+). Does anyone know what the issue might be? Let me know if you need any more info. Thanks in advance for the help!