The only thing left is the fuel pump. I know you replaced it, but I'm wondering if the cam lobe is worn thus causing the pump not to produce enough. If you think of it, at slow speeds, there isn't as much demanded of the fuel pump. At high speeds, more is needed.
Also, I wonder if the float is set properly. When you rebuilt it, did you have the right measurements for float drop? Maybe the carb is running dry because the float is set too high and shutting off the gas?
I would double check that and perform a volume test on the fuel pump.
Let me know what you find.
PS: You are sure there are no vacuum leaks, correct?
Friday, March 13th, 2009 AT 9:44 AM