I just finished 3 weeks of troubleshooting my cold-running TT 5 AT. Infiniti does not have the right part listed for a replacement. They only have one replacement thermostat when there should be two. Manual trans has a different opening temperature (clearly indicated in the G35 shop manual - see below) from the 5 AT. I was lucky to find a great little auto-parts supply store who did the research and found me the thermostat I needed.

Here are the details: