Just a thougt:
I had similar problem long time ago in one car. After months of tests, one new fuel pump, new fuel filter... It turned out to be a dirty fuel tank. It had a tiny crack and water and mud would get into it. Took the tank off, welded the crack and washed it with jet washer inside. Left it to dry for 2 days, installed it, cleaned all fuel lines, new fuel filter, fuel pump cleaned, fresh fuel and have never had the problem again with it.
I don't know, just a thought...