1991 GMC Sierra stalling and poor running

i have had some trouble as of late. when the truck is cool it will start and run. soon as it is warm the idling is rough ( it feels like the timing is off) and will stall. the truck then will not start. i checked my plugs and they are a little white on the tips of the conductor arm. i first thought that it was the ignition module for it wouldnt start so i replaced that, i also have swapped the coil. any ideas

thanks

travis