Its been a long time since anyone posted here but I am having the same issue with my stillen kit on my 350Z. I saw that some folks have replaced the extension harness' and the issue went away. I just ordered them today in hopes to fix this issue. Brutusvk, did you ever get you P1239 code to completely go away?

Yes, this is true.

Before stillen had the those maf and throttle body wire extenders, I was getting a lot of problems. i ended up making 4 new harnesses with no soldering, bought 4 connecters from Nissan, with the pins, haven't had a problem since.