help with hei distributer and coil voltage
 

i have bought a hei distributer for my 400 and when the motor is cold it runs very well but as the motor warms up it really struggles to idel properly and sputters out but seems kinda ok at higher rpms anything above about 3000 any ideas to help that would be much apreshated. my coil is also suspicious as it gets 11 volts while in run positions and up to 18 while running i wasnt sure if that was ok so i wanted to include that as well anything eles youd need to know to help feel free to ask and ill do my best to help you help me thank you.