Please Help Me!!

Hey I'm new to the whole Mopar seen. I came from the GM side. I have a 68 Barracuda with a 318 auto. I thought I had a fuel issue, replaced pump, tank, carb, lines etc. same problem existed. So I repaced plugs, cap, rotor, wires, coil, resisters. The car will start and run if you hold the key in starter position , but as soon as you let off that, to just run and acceseries, it dies. I pulled the ign. switch and tested it, and it tested fine. Anyone else had this problem. The car will randomly start up for a few seconds every so often, and I drove it for 30 mins last week before all this started happening. Thought about looking at the neutral safety switch next. Someone please lead me in the right direction.