I'd check two things first:
1. Is your starter overheating? If you have headers and they pass close to your starter, you can get heat soak, which will cause the solenoid to not engage when its too hot. Solution: starter head shield.
2. Do you have an electrical choke that isn't opening enough at hot temps? Remember the choke should be closed when cold, more open when hot. Solution: adjust electrical choke setpoint.