yer right Stoo -not a wiring or resistor problem - on a 12v setup, the coil will be getting just over 9v thru the resistor, full 12v when cranking
if it's not a weak battery problem [cranks well any time] then it's timing, as Moll sez - try tweaking the timing slightly and see what happens
could also be the carb is flooding it sometimes - floats do stick and these carbs are notorious leakers
what does "When cold I have little problem starting it" mean? hafta pump the pedal lots and give it full choke to start cold? = leaky carb and near empty float bowl - although most all Chevy's like a few pumps to fire right up, when warm it shouldn't take more than 1 pump, and if the timing is right it'll fire immediately
Bill