I bought a 2008 WRX that was lightly modified and I've been getting spontaneous bursts of what mainly consist of misfire codes(P0171, P0301, P0302, P0303, P0304, PFFFE). I say spontaneous because there's no noticeable change in handling or performance when the codes pop up. Up until now, I was attributing the codes to poor cold start performance in the subzero temperatures we've been experiencing, since the CEL only came on when the car was still cold and idling poorly. On my way home today, it popped on after accelerating out of a turn at operating temperature, as well as adding a PFFFF code to the previously mentioned codes. The only performance issues are at idle after a subzero start, the car idles poorly(I can feel the car shake side to side) and the aftermarket mods consist of: Equal length headers; turbo back exhaust(with two outlets); aftermarket intercooler; COBB AccessPort tuner(currently running the Stage 2 91 LWG map); COBB short intake; Stage 3 Comp clutch. For a short time, I switched to the non-LWG version of the Stage 2 map but switched back since, only with that map active, I got an O2 sensor voltage low error code that cleared and an H02S heater circuit high code that would stay on even after repeatedly clearing codes. The previous owner had also changed the spark plugs before I bought it. ~87500 current mileage.