1978 F250 charging problems
 

I have a 78 F250, I have a new alternator, 850cc battery, starter selinoid, voltage regulator, and cables. The truck starts up fine, then I drive it about 15 miles, and it wont start again. It acts like my battery is dead. I test the battery and its good. I jump it and its fine until I shut it off. I have replaced 2 bad alternators, and had the battery tested again. Its like there is a weird short somewhere. When I had electrical test ran, they kept telling me it was the ignition. I replaced it and its still the same...Any help would be great. Thanks