Fuel pressure
Definately check your fuel pressure. You are describing classic fuel pump going out problems. I have two 6.0's in the past couple of years that the pump start and stop working almost at random. The first one drove me nuts till I drove it for a week with the fuel pressure gauge hooked up.
When the fuel pump is taking a dump the trucks will idle perfectly, start great, and then randomly crap out once you get them under any load.
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