I have been also plagued with the temperature sensor issue, my 06 is now on it's 3rd one. There are still times when the temp needle just drops down to nothing for about a week and comes back to life on it's own when ever it wants. Last sensor was done by GM garage so that I was sure it was done right. Anyways, for those times it is dead I am getting supplies lined up to add an aftermarket digital gauge but am wondering if anyone knows if there is any plugs in a water jacket anywhere on my 3.9 that I can screw out and thread in sensor. I kind of want to get away from cutting top hose, measuring inside diameter and adding the coupler thing. This will also unsolved the mystery of what temp is it running when needle is just under normal or just over normal. Also got a glowshift trans temp gauge and also curious if anyone knows if there is a diagnostic plug / port a guy can unthread out to install this sensor as well? I have a in line brass sensor block coming in the mail, but would rather thread directly into the trans if I can.

 

Have you thought about trying an ob2 gauge? that being said if the problem isn't in your cluster it may not work either...