I don't know how relevant this is to the discussion, but getting back to the stone filter, they seem to be very susceptible to having water in the gas.

A few years ago, I got my old Jeep out of the garage to go on a four wheeling trip. I had put some gas in it out of a drum I kept in the garage, and unbeknownst to me, the drum had some water in the bottom (condensation??). The Jeep has a High compression Buick V-6 in it with a Holley double pumper on it. It has those stone filters in the gas inlets. I got down the road from my house and started having trouble with the Jeep. I pulled over and eventually diagnosed the water in the gas. The engine would not run on the primary circuit but would rev up when I cracked open the secondaries. I removed the filter from the primary inlet and couldn't even blow through it. With the filter removed, the Jeep ran fine (after pumping some of the bad gas out.

That stone filter is obviously not doing it's job and might even be a big part of the problem.

I know you say the tanks are clean, and the filter at the selector valve is clean, but it makes sense that you might be stirring up something (dirt, rust, filter sock parts, ???) when you fill up with gas. This idea is based on the statement that the problem seemed to reoccur after getting gas. I know it's a pain in the butt, but I think would be taking another look at the tanks as well as any steel gas lines. The ethanol in the fuel is rumored to be rather corrosive plus it is hygroscopic, which might account for the water plugging up the stone filter.

sounds like maybe the mechanic might be either lazy and dishonorable or incompetent?

YOU ARE KIDDING, RIGHT?

YOU CAN'T BE SURE OF WHO YOU ARE BUYING YOUR SUPPLIES FROM, HOW CAN YOU FINGER SOMEONE YOU HAVE NEVER TALKED TO???

Buick V6 power eh? ever hear of Jim Ruggles? was an old friend who built dominant busch grand national and inday car engines up until they outlawed those Buicks.