boost problem

[olly-civic]

dunno if anyone eles has had this problem but somtimes when i get up to 5000rpm the bigger turbo dont come in and you have to ease it back and bring it up again its a bit of an intermittant problem, someone told me i could have a split pipe, or a dodgy valve, but was hoping if anyone eles has had this problem if they could be more specific as theres one or two pipes n valves in there lol