You say that it happens once the vehicle is warmed up makes me think that your coil is going bad. Pull the coil wire off and see if there's oil coming out, also if the bottom of the coil is half moon shaped it's going bad, now you can not have either of these 2 issues and the coil can still be going bad. The resistor could be going bad as well. The pickup in the distributor could be going bad. I would check out the other 2 possibilities.