I doubt it is vapor lock-- EFI setups hardly ever get that, that's a carbeurated problem. If you're having issues on both tanks I'd say it's probalby not a fuel pressure issue either unless they both use the same fuel pressure regulator (which I am not sure about, I have never owned/worked on a dual tank truck before). If they use the same regulator, that might be going and causing the issue.
If that checks out OK, take a look at this thread: http://fordsix.com/forum/viewtopic.p...ghlight=shield
. It looks like there is an air-cooling device which is supposed to prevent heat soak. Do you have one of these in your truck, and is it operational?
If not, you might want to get one or a heat shield off a newer truck and put that in there...see what that does. Make sure too that your computer itself has a heat shield.