Driving at hwy speed truck felt like the key was turned off for 1 second?

If this is a 7.3L diesel then I suspect, we have a fuel issue. It is not going to be an ignition issue because there is no ignition system. The fuel is combusted due to pressure.

Basically, when this is feeling like it is cutting out and then starting again points to a drop in fuel pressure, so we need to start with this:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

I am attaching the specific process below from the manual that will help with this.

If the pressure is dropping, then this could be a pump issue or even a PCM issue so let's start with this and go from there.

Please let us know what you find. Thanks