Does the SES light come on? That usually happens with a real misfire.
You have to remove a lot of parts to fix a valve cover leak, so it's possible there is an air leak somewhere. It's also possible that relearn procedures need to be done.
Try the instructions in this DIY. Technical Articles: G35- How to:Reset ECU, Throttle Pedal Release & Idle Air Volume Learning
http://www.nicoclub.com/archives/g35-ho ... rning.html