In this thread I posted that I had the slight yo-yo surge on my 2006 MCS >>https://www.northamericanmotoring.co...957#post861957

I found out what the problem was... I have the JCW airbox and there is a rubber vacuum line that comes off of this to keep vacuum on the flap control and open at 4,500rpms. When the dealer installed the kit they put the factory "T" on the back pressure relief valve (BPV) and then run a small (6") section on new line with a barbed white plastic connector on it to the JCW rubber female connection. Guess what? I was checking out my engine compartment with a flash light last night (looking for any 948 mile visuals) and it came undone! I plugged it back in this morning (for good) and HOLY CR^P - smoooooth as new. No more vacuum leak or surging. Done.

Back when we took delivery, I noticed on the drive home from the dealer in 6th gear on the interstate that the slight surging started and when I got home searched NAM archives to find that it was/is a problem. However, 800+ miles later, I found the culprit. Power seems to be better from 1,000 rpms to 3,000 rpms too and NO more surging.