Welcome Kitch.
Boy you're thorough. Nice work. Electricity seems fine, then, go onto the tubes. Check for leaks. My guess, considering all you have done, is going to be way simple. Start with the fuel line hoses that feed the dfpe. I've found mine full of white powder. Remove, inspect and replace if necessary, all the vaccum tubes.
Karl.
You have plugged your codes into the "Search this forum" box, right?