Issues Starting My Camaro?

ok so this is really confusing me but its probably something silly so i have a 82 camaro with a 305 it just started not wanting to start after i left music on after shuting it off so i thought ok i fryed the battery so i went and bought a new one because the old one wasnt holding charge it seemed because it wouldnt start the car so.. ok new battery car starts fine i go for a cruse come back turn it off then try and start it again same thing wont turn over .. so then i go ok altnator is shot.. so since i didnt have anymoney at left for a new one i let it sit then today i went out i was about to jump start it to test the altnator BUT i thought id give it a go to see if it would start and it does? and its a really strong good start.. so i took it for a drive came back turned it off tryed to start again same thing sounded like it was struggling to start and wouldnt go as if the battery was dead and before i went i tested the altnator it says 14.5 ok so i think the altnator is good…? so would it be the starter motor? someone please help !!!

Suggestion:

As heat increases, so does resistance and performance. The starter is probably worn out and old.