I always have the service guy call with what is wrong and a price once they find the problem. that way things like that dont happen. I dont know about the 04 GTP but my 00 thermo is a breeze to change. 10-15 min and Im done.

Your problem sounds like its the same as on my 00, the PCM isnt seeing the car heat up FAST enough so it throws a code, and I have a 180 tstat. My car was doing it intermittantly depending on the temperature outside so I took it to a dealer and asked them to check it for me.

ZZP has an ESTAT module now that takes care of this but still allows the gauge to read properly. Its on my to do list

http://www.zzperformance.com/zzp/products/electrical/estat.htm