bismic gave the great advice. same thing happened to me on my 05, 6.0L. It took my mechanic 6 months and 6 tries before he found the chafing wires. I as doing 75mph on a freeway and bam, lost power and luckily was able to weave through traffic. this time it did not start and needed a tow. after tow to mechanic it started so he kept the truck for months and sure enough it happened to him. that's when he found it. it was 9 years ago and I don't live there now or I'd go ask him where the found it. but chafing wires was the problem.