All, I bought a used (well of course used) 2008 Dodge Ram 1500 4.8 V8. I had the exact same problem while driving 30 miles away from the dealership after purchase. Was doing 70 on the highway, and it went completely dead (as if I had shut off the truck) slowed down, and finally came to a stop. I tried to start it in neutral while driving, but the truck was already done. I spent the better half of a month locating the problem I had. I'm going to tell you everything that I did (this was an everyday problem by the way, whether I was in park, driving, stopped at a stop sign).
1. Replaced the Crank Sensor (did not fix it)
2. Replaced the Cam Shaft Sensor (did not fix it)
3. Replaced the Ignition Switch (did not fix it)
4. Added on the next 3 fill ups of gas, a can of Seafoam (did not fix it, but it did run better)
5. Took it to the car wash and cleaned the tires with Clean White (that definitely didn't fix it, but I wanted my wife to at least enjoy the beauty of this beast, which I have always loved the way a dodge truck looks)
To give you some background on my new to me truck, it came with 129,345 miles on it (not bad for an 08). It has a cattle guard on the front that would withstand any deer attack. It is a Quad Cab, 8 foot bed, and silver. I loved this truck from the moment I saw it, and the price was very good.
I decided to do the regular 100,000 miles maintenance on it. I ordered all 16 spark plugs (intake 8, exhaust 8), 8 plug wires, and 8 Coil Packs. I took a day off of work to do this, and what I discovered right off the bat, I couldn't get the first plug out (it started to turn, and then tightened back up, I put it back and left all the spark plugs alone. I can't afford to rethread or get one out if it broke, I'll hire someone for that.)
I did go ahead and replace all the coil packs and the plug wires. One of the coil packs had a white arc looking burn on the boot of it, which I have to assume may have been the entire problem. I ordered all of this off of Rock Auto, and for the past 2 weeks, my truck starts better, and it has not quit yet. It has had every opportunity to do so, with the humidity, heat, cool nights, regular Missouri weather.
It may not fix them all, but in all the threads that I have read, and research on the internet, no one mentioned replacing the coil packs before. That and the plug wires cost around $180, and they were mostly easy to do, minus the one closest to the steering wheel.