When i bought the 2002 sti it had front end damage and minor parts on engine were broken and cracked. i replaced crank pulley it was bent,timing was still ok but i replaced timing belt and re did the timing.intake manifold was cracked so i rplaced it.altenator.power steering pump.and air con was replaced.also replaced exshaust headers they were damaged aswell.. mods that have been done by previous owner are ej25 forged rebuilt motor,bigger fuel pump,bigger injectors,exhaust headers and full exhaust, bigger oil cooler,52 mm radiator, aps sr55 turbo,bigger air intake,ecutek flashtune.. so you think fuel pressure regulator do you mean the one thats next to fuel filter or else. ??
I recently had the same problem with my jeep. I removed the throttle body and cleaned it and cleaned the IAC. I don't know whether the subies have throttle bodies, but I imagine that they would. It seems to have fixed the problem, and now with my "custom" intake, it kind of whistles like I have a turbo.