This is what worked for me in regards to the hot starting problem:
New battery cables
Delco 1000 CCA battery (installed in 1995)
Heavy duty solenoid (part# GME100C, from PY, $35, installed a few weeks ago)
Solenoid heat shield
I have an otherwise stock Delco starter, with a NAPA starter drive that I installed a few years ago. This is the same starter that was on my car when I bought it 7 years ago. The car starts fine (stock 400, 350 horse, stock exhaust manifolds, '68 GTO), hot or cold, with the above equipment.
Try the heavy duty solenoid and spring; I'm sure that'll solve the problem.
