Help! Slant 6 keeps dying

I have a 1973 Plymouth Valiant with a 225 Slant 6 automatic. When I drive 20 or 30 minutes or more highway speed (about 75mph) and then get off an exit, as soon as the car slows and I let my foot off the gas, the car dies. It doesn't sputter or shake, just dies. It starts back up but you have to keep it moving or it will die. After it sits for about a half hour or more it will run fairly normal but if you drive it long it does it again. If you put it in neutral when this happens it dies too. I have no idea why. Any ideas?