i would start by checking the intercooler piping to see if there is any rips or loose clamps.Forget about the spark plugs for now because they wont cause your car to run at 300rpm. You may also want to try and adjust the BISS. Please do a search on google or something, i dont feel like explaining it. Better yet, here ya go
http://members.shaw.ca/dsm.1000q/Eng...er/2G/biss.htm