first ever scooby

ever since i had my cobra 5 1/2 inch back box fitted it jerks down the rev range to the sound of the popping if that makes sense?! for example : if i'm in 4th and i take it to 6k, ease of throttle, the first 2k drop in revs is continuous and smooth, then about 4k the pops and bangs start and the jerking matches the popping. pop,pop,pop,bang,pop = jerk,jerk,jerk,nothing,jerk. very strange. i know turbo's require back pressure so is the back box not allowing enough? would a pipe that goes from the turbo to the back box in one piece eliminate this? any suggestions guys? cheers.