My first motor home was a 76 454 powered Sportscoach. I had the exact problem. I fought the darn thing for a couple years trying to figure out what the "exact" problem was. In "my" case it turned out to be the wrong mechanical fuel pump for that specific motor. I was told several times the particular pump I was buying at the time was the "correct" one, they were all wrong. Each time I changed the fuel pump the coach "seemed" to run ok.........for a while. Then the vapor locking symtoms would return. I owned the coach for 6 years and lost count of how many "correct" fuel pumps I installed on it. What the problem ultimately turned out to be was the "wrong" length of arm on the fuel pumps I had been buying. The correct pump has an unusually long arm on it the runs off a cam lobe as compared to others I had bought. Go to a parts house and have them pull several different pumps prescribed for that motor and see if you can't corner the correct one. Another thing I ran into during this process is, GM's design of the fuel line location running from the fuel pump pretty tight up against the front of the engine. It got hot very easily. I removed mine bought a new section of fuel line and routed it away from the engine block, I also wrapped it in an insulating sock. Between the fuel line location and the longer arm fuel pump I cured the vapor locking problem.