Boy, does my son's slant run crappy!

My 63 Valiant did this and ran good till it got warmed up then it just barley made it home, It was my coil as soon as it got hot it would brake down.
The very same thing happened to my truck 83 318 I changed the ecu and coil and it fixed the problem

Weird that the coil and ecu was fine till it got hot.