Matt,
I've been dealing with this same problem for several years now on my 71 440 Cuda. My symptoms are exactly the same as you described. It started to happen on my car after it sat for a couple of years. Once I saw that I had this problem, I switched over to a high output starter. This didn't help. Next I installed a new Optima battery. No change. My timing is locked at 34º (same as it was before the problem), and I've been retarding the timing on startup using an MSD retard box. Note: Before this problem I never had to retard the timing on startup (10:1 compression). So, the startup retard has a minor positive effect, but does not fix the problem.
I talked to a friend of mine who is a very competent mechanic, and he said that there is a voltage drop occurring somewhere in the starter power supply (i.e., the battery cables). He said you can determine where the voltage drop is occurring by measuring the voltage across the cable connections (battery-to-ground, battery-to-starter, engine ground-to-frame) when the engine is warm, and the starter is cycled. Have a person help you, he can run the starter, and you can connect a voltmeter across the cable connect points and measure the voltage. If there is a difference in voltage of something greater than, say, 0.2 volts, then you either have a bad cable or a bad ground/connection. You can narrow down the location of the problem by putting the voltmeter wire on the battery post for one test, then on only the cable attached to the post for the next test. This would check the cable-to-post connection.
Anyway, look at your battery cables for corrosion, cracks, loose connections, etc. This might be a quick fix if you see something obvious.
I haven't had time to track down a buddy to help me with testing my battery cables, but I think I am going to go ahead and install new cables this weekend anyway. I hope it works!
Good luck with your diagnosis, and please post your solution as it comes to you. I'll post the result of my cable swap as well.
Jason
