Could be your timing is too far advanced. Have you changed it lately. Had the same problem and when starter got hot it would barely turnover. Kicked back timing and it was fine.

 

i tried to start the car last night thinking the heat was the problem and it still didn't start. but now when i turn the key it's a totally different sound. almost like a buzzing. could this be the starter?

 

does anybody know what this buzzing could mean?

 

buzzing is the majority of the time your solenoid. and that would be a good reason your car only starts when it wants to! lol! check your solenoid and try and make some sort of heat shield from the starter. but i'd be willing to throw a 6'er on it that it's the solenoid.