1969 dart loses spark once it warms up

Hwy guys hope someone can help I have a 69 dart with a 360 it has MSD ignition (6al box) with a msd blaster coil and the distributor has been converted to electronic from points. It will start up fine when it has been sitting once it warns up it will lose its spark and stall and not start till it cools down again any ideas?
Your Blaster coil is bad.. I had a similar problem and after doing this and that I tested my coil with a meter and the OHMs were way out. A new one eliminated my problem. Just to be sure, test your coil.