Boost fluctuation
 

hi all
i've got a v3 sti which i was able to open up for a change on the way home from work the other day and i noticed some boost fluctuation mainly in 5th gear. it seems to happen more around half throttle and say about 3- 4000rpm as i put my foot down a bit the boost hits just under 1.3 really fast and then comes back down then up etc etc. soon as i put my foot right down it levels out better
the car is mapped by jgm to 1.3 bar with an apexi fc so it's not overboosting. any idea if this is just the apexi stopping it overboosting or if maybe the bcs is on its way out ?
thanks