Hey umm it can also be the CQC its usually breaks if you press and hold the clutch often you can find a heavy duty one here
350z 370z heavy duty CSC Concentic clutch slave cylinder funny last time i checked it used to be $250 i guess the demand went up. Or maybe if you have some extra $ laying around and you would like to get a lighter flywheel and clutch and and an even better CQC i suggest and i bet every one here would 2, get one of these
Z-1 Performance Automotive Services Inc. - ATS Multidisk Clutches 2009 + 370Z (VQ37HR) yeah its a little expensive but if you love you're ride like me you would get one 2.