Also, I would disconnect the fuel feed to the carb stick it into a container and run the pump (electric in this case) for one minute and the measure and calculate the max fuel flow your system gives you.  This won't prove anything is wrong with the carb but will at least show you are getting more than 8.7 gph fuel flow to the carb.  If you are not getting at least 18-20 GPH fuel flow I think you have something  in your system that needs fixing. 

 

About all I can think of.

 

Ed A

 

I'm sure someone mentioned this, but did you check the fuel tank vents? you said you could produce the problem on the ground? Try doing the run up with the fuel caps off. If all is well you have a tank vent problem.

 Also a friend of mine had a chevy luv pick up, actually a re-baged Isuzu I believe, and he had a problem almost identical to yours. He had done a re-build and put everything back on in order. The truck would run fine until he went more than 55 mph for about 3 minutes. It would then stumble, but continue to run. His problem was the carburator had a return line built into the banjo fitting to prevent vapor lock I presume. The banjo had two identical lines (3/8 barb) on the outside but when you looked inside there was one .300 id hole and one .062 id hole! the small hole was supposed to be on the fuel RETURN line and my buddie had inverted the fitting and switched the holes. Everything looked PERFECT, (he even took pictures before he took the thing apart), but the small hole couldn't keep up when a high continous flow was needed! The problem nearly drove him bonkers. A stupid design really; should have had two different size lines.

Bill Jepson