slant six starting problems

I have a 75 Dart with a slant six that is giving me problems. It was starting fine and when I drove it for a while, I turned it off and it would not start until it cooled down.
Now yesterday, it would not start at all. I changed the coil ater looking at different posts and still nothing. I changed the ballast resistor and it started up but stopped after running for about five minutes. I got it started again and let it run for about ten minutes and it died as if someone shut the motor off.
I had an extra ECU so I changed it and nothing. The same with the voltage regulator as I was getting desperate.
I am getting fuel, spark at the distributor, and now looking for any suggestions.
Thanks in advance.