Still stumped

So I'm out of ideas. The M1008 will run fine for about 15 minutes and then start to surge and stumble. No problems at idle. And it actually ran fine for two days (80 highway miles) with no symptoms. I've replaced all the rubber lines, all the even slightly rusted fuel lines, new tank, new lift pump, been through a few fresh filters without a change in symptoms, clean air filter, cleaned the return valve in IP, timing checked out ok on pump and it was rebuilt recently. I've been to two shops now and no one can find the problem. The truck will run fine in town at 30mph for a longer time, but the symtoms come and go otherwise. I could really use some direction.

Time to check the pressures at the injectors. Bad IP or injectors...