Zoom,
How close to the headers is your starter? Heat from the headers has been known to cause this on a lot of these cars. Also where is your battery and what size battery cable do you have? I had to use a high torque starter but my compression was to high for the stock starter to even start mine cold. I also used some reflective heat material to insulate my starter from any header heat. You could try backing the timing off just a little and see if that helps, but I would suspect heat affecting the starter more. And if it is the stock starter then it will be affected more than a high torque one.
Ron
