'86 D150 dies while driving or idling
 

I have recently changed plugs, wires and did an oil change. The problem is it will die while idling or driving down road. If I put it in neutral or park it starts right back up, no problem. Going up hills it loses some power but not completely. What is the most likely problem? Any help is appreciated, got to travel for funeral in a week, so need this fixed soon!