A very common problem with GM starters. I used heat shields, changed starters, etc., but nothing worked. The problem, especially if you have headers, is that the heat soaks the starter and the wires. I tested the voltage one time and only 6 of the 12 volts was getting to the starter because of the heat. The solution? Do what Ford did for years, use a remote solenoid. Mine is on the left fenderwall. Now, no starter problems. Here's a link explaining the solution.
http://www.oldengine.org/unfaq/solenoid.htm