Thanks for the suggestion. I do not understand how the tank system works for a diesel of this vintage.

I wish I had thought of removing the cap instead of changing the secondary filter. I have very little time to test for reliability now. Instead of leaving it off, I will drive until the problem occures and then remove it. If recovery is immediate I will know instead of assuming that all is well if it runs for xxxx miles. It could be fine for 200 and then fail in 50. It must relate to tank fill and a bunch of other variables.

I will re-read the shop manual re the expansion tank and vent valve and try to figure out what is supposed to happen and how to test it. It was pretty confusing the first few reads. If anyone has a clear simple explanation as to how this system works and how to test it and what components fail, that would help. I like to follow logical diagnostic procedures as compared with just replacing parts until the car works.

It does make sense that the fuel pump is not the primary cause of this problem, but does anyone know whether extreme variation in output pressure is normal when measured with a standard pressure gauge? It seems to me that the relief valve should hold a somwhat steady pressure. I have not actually found the relief valve yet but will continue looking.