Starter's heating up. It's very common with older starters and solenoids, with headers and stock starters and solenoids, and on cars missing heat shields.
Most of the time replacing the solenoid with a new heavy duty solenoid will fix it, but mine did what yours is doing and I replaced both the starter and solenoid. It works perfectly now.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
