could be a fouled up plug, faulty coilpack or faulty ignitor.
check you spark plug condition.
to check the coils:
when it sounds like a wrx, unplug you coilpacks one at a time, if it gets worse when you unplug one of the coils its not that one, if there is no change in the way engine runs then that coil may be rooted. then swap that coil for another, if the problem moves with the suspect coil then its the coil, if it stays in the same position (on the same cylinder) then could be the ignitor or the wiring to that coil.