I suspect you are setting up turbulence or a swirl inside the base of the scoop at speed.
Back in the 1960's - 70's Chrysler published plans for their ram air hood scoop used on the cars like Sox and Martin
http://www.gilchristbnw.com/GPC/DR/PS/S%26M7201.jpgThe scoop was called a Bowman scoop as I recall. It was important to have the right dimensions above the carburetors to avoid the air flow turbulence screwing up the air pressures in the float bowl air bleed tubes and the throats of the carburetor. As I recall the bottom of the "duck bill" on the scoop needed to be about 1-2 inches above the top of the carb so the air would settle down and dissipate turbulence.
I assume you have a block off plate on the bottom of the scoop that only exposes the carb bores and air bleed tubes to the pressurized air in the plenum of the scoop.
If not, you could be getting hot air coming up from in the engine compartment due to pressure changes in the engine compartment at speed. This could cause some bores to see cold ram air from above and others pulling in hot air from the engine area on other screwing up the mixture once your speed gets fast enough to cause major pressure differences under the hood and in the hood scoop.
If it is turbulence a couple layers of screen or as mentioned above an air filter box could break up any air turbulence and equalize air pressure over all the carb throats and float bowl air bleed tubes.
If I can find my old direct connection book that has those plans I will try to post the info they had on that scoop design I am sure the modern scoops need to address some of the same issues.
