Just fyi, my dealer replaced the speedometer assembly under warranty. The very same wknd, temp was in the mid-90's. I'm happy to report the problem never came back!

 

I live in Chicago area and have a 2007 G35x that has the same problem. This past summer, when the temperature was in the upper 80's and lower 90's, I noticed that when I turned ON the car, the RPM guage will read 3000 or 4000 while in Park. I noticed this problem almost everyday after work hours, around 6:00 pm on my way going home. Most dealer's service depatments are closed by then.
One afternoon, when this happened, I took the car into Lisle, IL Infiniti and showed the RPM. They attached some equiptments to the car and were not able to read any fault codes. They told me that the vehicle's guage cluster is faulty and must be replaced but since they did not have a loaner car to give me, they asked me to come back. I told them that this is an intermitten problem and they said not to worry and they have it documented. I went back about 4 weeks later to drop off the car, but at this time, they said the if they can not reproduce the problem, they will not replace the cluster. Well the temperature in Chicago area in no longer in 90's so the issue is not re-producable. To make the long story short, they refused the fix the cluster they witnessed not operating. I have since contacted Inifini USA and they claim that they have never heard of this problem.
I encourge all of you who have invested a lot of money into a vehicle to contact Infiniti USA and Canada and let them know about this problem. The cluster's cost is $1000.00 and once the warrenty is done on the car, it is coming out of our pockets to get it repaired.
Here is Nissan North America's number (800) 647-7261
Lisle Infinit in Illinois must have the worst customer service I have ever seen in a luxury car's business. They operate the facility as if they are setting Kia.

Thank you

 

This is interesting. I live in Tucson AZ and it's been 100+ all summer. I haven't had that problem at all.