Won't stay running under load, why?

About 1 month ago I asked a question about my '77 e350 motorhome. Since then I found one asked by kprince65 on 8-16-11 mostly describing my problem. I bought the rv in '06 in illinois, problem arose about 500 mi. From where I bought it. Rv has fresh rebuild (came w/1yr warranty card) new electric in line fuel pump (not in tank) and new coil. Not sure about block yr. Has un-machined boss for mechanical fuel pump. After market pump rated 4 lbs. Psi. Last time I moved rv, problem got worse. 35 mph at best, had to drive w/dog house off spraying ether in carb at stops. What's wrong w/this thing? Any help much appreciated.