Just a thought, do you have access to a 4 or 5 gas analyzer? With the sniffer in the pipe the results may point you in a direction. Also what is the fuel pressure? Have you pinched off the the return line to see if there is an internal leak in the fuel system. You said the plugs look a tad rich, that should be your indicator. Have you checked the ignition system? Again the 4 gas will give you direction. These trucks are also known for the coil arcing. Have you ohm metered the igition wires. How old is the precat O2? It isn't a bosch unit by chance? You may have to start with the basics and go from there. Sometime the obvious is overlooked.
Just a suggestion. I have one of these trucks and I really have suspicion of the spider especially if there is a lot of miles on them. They do some real strange things.