Stalling or something like that, on mid rpms

[Anemisthras]

Hello. I have a greek version of subaru impreza 2004 (1.6 turboed by the greek subaru's retailer). I have a strange problem, when the car is used it seems that it cuts power from the gas pedal only between on low rpms till 3000 rpms. If the car is cold this is not happening, only when the car is been used for a while in a day, or in a traffic for example. I have changed spark cables with oem ones, i have also changed the ignition coil, we have checked the plugs, but the problems insists to occur only when the car is used for a while inside the day. If i use it only to go to work for example like a five minute drive and get back home after 8 working hours,the car is fine. If i say it correctly it only does that when the engine is not cold. It drives me crazy because is not working as it should be. We also changed the ecu, nothing changed. After 3k rpms the car is fine. Any suggestions?