does it have a pod filter or oiled k&n panel filter, if so ditch it for a stock paper one & replace the air flow meter with a brand new genuine one, these are by far the weakest link on the my98/99 model and cause the problems you are describing, previous owner could have cleaned it & it ran good for a couple of days

the coilpack on this model is a pretty rigid unit & there is only one that does all 4 plugs so i doubt that it would cause a misfire, changing the plugs won't hurt but the damm afm's on the early wrx's are so so shit