I ain't no mechanic but from advice I've been given and what I've read it's a common problem with RB engines when stage 1 mods are done like freeflow exhaust, BOV, induction kit etc and there could be allsorts of causes. Things to check in no particular order are boost leak, plug gaps, coilpacks and amp, coilpack loom can go brittle, clean the afm. Then start thinking of a remap because it's the rich and retard problem caused by the mods or if your not going further with mods then an air/fuel controller should do. I've just fitted an Apexi SAFC2 to my R33 to cure the same problem and it revs through to 8000 rpm beautifully now :bigsmile: Hope that helps.