Slant Six Bogging Down!

My 69 slant did that on a cross-country trip in 1989. It would start stumbling above 55 mph on the flats and less up-hill. Seemed worse in the afternoon and especially as we drove into NM on a hot day. I found the fuel inlet hose at the fuel pump was soft and thus collapsing at higher flows. Swapped in new hose in the parking lot and drove the rest of the way to CA at 65 mph, and smiling. One of the few times my wife thought I was a genius. Our 2nd boy might have even been conceived that night. That was about the time they started putting ethanol in the gas, especially as we went west. Use only "fuel injection" hose today or will have problems. Also check the fuel hose at the gas tank. But could be any of the fuel problems mentioned above. Too much spark gap can do the same, but you changed plugs.