2flash2furious

ive got a o1 impreza bug eye which has been off the road for the past two months waiting for a new gearbox, just fitted new gearbox and find now that between 2800-4000 rpm has really bad flat spot. power backs right off then cuts back in harsh and off again, was fine before came off the road, checked all hoses to turbo and cant find any leaks, any ideas?