Hi can anyone help me ive got 2000 mk4 golf gti 1.8t. 150bhp Sometimes it boost and then it doesnt boost i have changed coil pack, diverter valve, n75 valve, i have checked pipes best i can as i havent got an air line. when i put new n75 valve on could hear swerling noise so put old one back on and no swerling noise but what ever i do still no boost properly didnt think it would be boost leak as still have boost that works when it wants but im new to turbo cars please can anyone help