Symptoms:
1. During idle, the engine makes one sudden, jerky vibration(quite strong) about every 15 seconds and when this happens, the rpm indicator would drop then come back to norm(650), this is after the car's warmed up and sometime the frequency of this vibration varies...

2. When driving at around 45mph in 6th or 7th gear, if you push 1/3 of the throttle(or max amount before it downshifts), this jerky vibration appears again but about every 0.5 second as engine rpm climbs from 1200 to 1600. It almost feels like the transmission is slipping....however I haven't been able to reproduce this symptoms in manual mode(shifting between 5, 6 and 7 while almost flooring the gas at 45mph) so I don't think it'd be the transmission.

Background:
2012 G37 sedan base, everything stock, no tune
New spark plugs 4 months ago...
***DIY changed valve cover gaskets right before this happens(most likely caused this)


What I've tried to fix this:
1. Cleaned PCV valve(it was working fine but I still cleaned it any way, issue still exists)
2. Cleaned throttle bodies and MAFs twice along with re-learning procedure (issue still exists after every cleaning)
3. Disconnect battery(this worked like a charm--car has never been so smooth....but only for the first 10min's driving....then it went back to this issue again)

My thoughts on this:
1. No check engine light yet, and I've thoroughly checked every wire, vacuum hose and even the fuel line I disconnected when changing the valve cover gaskets, haven't found any leak or wire left unplugged or anything abnormal, I think it could be the coil-overs but there was another guy on this forum who had similar issue and changed all coils and plugs but issue still exists so

2. I've flushed the transmission fluid 3 times in a row a year ago and I'm sure it's at perfect level, the car shifts well(not as smooth between 1 & 2 but anything up is great) so I'd probably exclude tranny out for this issue

3. It drives fine and smooth most of time, throttle response is good but again this issue happens in that specific scenario (1/3 throttle when cruising at around 45mpg in 6/7th gear) and gets me really paranoid since it's impossible to ignore....however I still want to try my best to fix this issue (no dealers/shop, don't think they help at all, just by reading the FSM I know more than them)

I could really use some help.....it's driving me nuts! I'm even thinking about this in my dreams
What do you guys think? What should I try next to solve this/further diagnose this issue? I can try to record a video for the issue too if that helps

 

How did you check for a possible vacuum leak? Did you do the smoke method? How many miles does your G37 have? If you did a complete flush at a very high mileage that could be a reason for symptom #2.

If you believe it is the valve cover gaskets double check the injectors, PCV hose/valve, again to be sure. Even small cracks and cause an issue like this. Sometimes the valve cover gasket can crack when you're putting everything back together. My hypothesis is that you probably cracked the hose.

 

Join the club. They all do this. It has something to do with the throttle bodies and MAFS. Try cleaning the throttle bodies and MAFS, but more than likely you might have to get new MAFS. Go OEM although very expensive aftermarket ones will make it do the same thing