i had the same exact problem with my 04 sti, it has something to do with the avcs solynoids. I took it to my mechanic and he fixed it, he said it broke and some got caught in the fuel pump.

hi there, can you help on something?? I have my STI 06, engine check is on steady and my cruise light keeps on blinking at the same time, i have changed my 4 spark plugs and one of the coil packs & fuel injector in which CYLINDER 4 code is reading error by the scanner, but still yet idle is rough and my engine shakes erratically, do you know what would be the issue or is there anyone have the same problem, my mileage 129500 at most. Let me know what would you advice to do. Thanks. Mr.NiceGuy24 from HAWAII