Pigfrog

I've had my MY03sti ppp for a month now and up untill a few days ago was running fine. I have now noticed that whilst accelerating hard in any gear there seems to be a dip in power between 5500 and 6000 rpm, where the car feels like its just stoped pulling, and then the power comes back on. Any advice regarding this problem will be greatly appreciated, and just to mention that i'm not very technically minded.

Cheers.