I've been having sorta the same problem. 02 WRX 130k. I replaced my A/C compressor and put a new belt on (which there wasn't one before I got the car) and it started this really bad missing. I changed the plugs and I ran fine for about a week and now it is doing it again. WOT from about 4-5500 RPM. Changed fuel filter and that didn't help. Only code is P0103 or MAF. I'm just as stumped as you.