bouncing idle

I just got my car back from the shop where I had a Zex wet kit and a VAFC installed and dyno tuned. The trouble is every once in a while I'll get a bouncing idle. It won't happen all the time but at least once a day. It'll go away after a bit or if I shut the car off and restart it.

I called the shop, they said it could be the plugs are fouled out from running too rich(recently changed them to NGK brk7e about 2-3 weeks before nitrous) or the O2 sensor is going.

Could it be the VAFC causing it? But if it is wouldn't be do it all the time? What do you guys think? Maybe they just f**ked up? Thanks.

see thread on "idle when cold etc." page 2... the iacv/eacv cold idle control valve can and does cause this. this should tell you all you wanted to know and the some.

check out "idle problem" page two first"