revs surging

i have a 99 323 ci and ive just put a aftermarket backbox on now it seems surge in the revs, driving steady its fine but say i boot it at 2000 revs it seems to jump rather quick to 2500 then back to 2000 then its fine all way up the rev range, does this at any revs every gear but only first time i give it a boot full

any ideas any1? hope its not the clutch