1978 Ford F250

Sound like you've covered the electrical. Are you getting spark? How about fuel? Have you checked the fuel pressure?

Check/Replace gas cap...

I just had this problem on my 92 e-150. I took a brand new spark plug and removed #8 wire as with the cover removed it is right there. slid plug into spark plug wire and taped the plug so the threads ground to the exhaust manifold. Started cranking her over and no spark. went to the fuse panel-ok-then to the relay. honked the horn then swapped the horn and ignition relay and horn would not work. So bad relay??? went to check #8 plug for fire but engine fired up. drove around the block and it shut off. no fire again. put old relay back in and it fired right back up. Finally bent each one of the 5 prongs on the relay and it fired up ,same with horn and it works. over time the female connectors inside the power distribution box corrode or heat wreaks havoc on them and they distort. I had to remove battery and tray,disconnect wiring harness which leads to the ditribution box and remove the box itself.the connectors are all 1/4" female "crimp" connectors and are removed with a tiny screwdriver depressing a small tab which holds the connectors up in the box. check for corroded wires and splice new wire and female connectors then reversed the entire 20 beer act and a week of new colorful words and she fires right up.The dome light works now and reminded me off the old "ford has a better idea" commercial!!