My 2008 G37S coupe had an issue with the transmission. At the tail end of a 75 mile trip I hit the gas and it dropped into neutral instead of kicking down. I tried it several times and it did it each time. On the last leg of the trip I used the paddle shifters and it shifter normally 5, 4, 3, 4, 5. The only new thing is I just had the transmission flushed at Bradshaw Infiniti. The service writer suggested I try it later after the car had cooled. i drove it and it shifts and kicks down fine.

 

I'd check the fuild level since you just had it serviced. If you dont know how: make sure the tranny is up to temp, the car running, in park pull the tranny dip stick n see where the levels at. Add some if it's low, if not I'd bring her back

 

When I got it home I checked it. It is dead nuts on. Looks clean (obviously). I drove it around 9PM and it drove and shifted fine. It could be that my manually shifting it corrected it.