hesertation under throttle

[scooby 887]

hi guys new to the site and hoping somebody out there can help. when i accelarate you can feel a slight miss or hesertaion mainly i notice it in mid range its driving me mad ive changed the plugs new fuel and air filter ive fitted a bailey dumpvalve but the problem was already there before i put that on. ive had similar problems with a classic sti and it was coilpacks but ive taken this to subaru and they assure me everything is fine even though they agree its not running right. The car only has 28000 miles and is very well maintained no other mods except sti back box.Subaru have done a full ecu reset which seemed to work for a day but i was sure it was still not quite right then and a day later you could feel it back for sure. Any ideas would be great as subaru have had it for a week and they still have not got a clue.To me it feels like ignition under load
thanks
paul

[SeattleJeremy]

What Model, Year, and Market are you in?
What do you mean by STI back box (catback?).

[teflon_jones]

Yeah what exactly is a back box?

[scooby 887]

sorry my bad its a wrx bug eye 2001 mdel and by back box i mean the rear silencer last bit of the exhaust system. im in the uk
thanks

[Anub1s]

Time to check for leaks and cracks in intake piping. When your ECU is reset, it resets all of the stored or "learned" values for things like fueling and knock correction in timing. If it learns values during events that may not always happen, this can cause the hesitation. Cracks anywhere in the intake tract post MAF can cause these kinds of issues, so that's where I would look first, especially considering you've replaced filters and spark plugs currenlty which are even more common of a cause.