You need to get the codes read and see what they are. Do you have a boost gauge or any way of telling how much boost you were/are getting? Traction control shouldn't have anything to do with it.. besides, 2002 WRX doesn't really have traction control. It is possible that it overboosted and hit fuel cut because you aren't tuned, but that wouldn't continue to cause problems like rough idle or anything. If anything, a tune would fix it. Most Subarus require a tune to run an aftermarket downpipe, your's does not, although I would highly recommend it.