Mustang hesitation help?

I got a 66 mustang with a built 347, its got a new msd blaster coil a msd 6al new msd wires and plugs, msd distributor, and a new electric fuel pump, all of these are new. The car ran fine for at least 6 months. The car idles fine and drives smoothe, all the way up to full throttle. It will burnout at wot fine, but if you let it go wot for a long time like maybe 20 secs it will suddenly fall from 6000 rom to maybe 3 grand for at least maybe 4 or 5 secs. This car doesnt miss or anything, but what is the cause of this? Could it be something in my carb? Thats the only other thing I think it could be, and if so what? Also I have to run premium fuel in here which I always do.