[swl]

Dave's advise is as usual solid - do your trouble shooting logically and you will save time in the long run.

However ...

Since you say you don't get along well with wiring diagrams and your symptom are identical to mine many moons ago:
Replace the 25 amp fuel pump fuse and clean the contacts on the fuse holder.

I can't really explain why but corroded terminals on the fuse seem to be heat sensitve. Driving along and it poops out. Cool off (or bump the fuse while poking around) and it comes back.

And after that fails go back to troubleshooting the right way.