[PbFut]

If you are getting a moderately correct temps while running at speed, and heating up at idle at a stop light I am inclined to think it is something to do with the fans. You have two fans. Are both running? Is it possible you bumped one of the leads when installing the Tstat. I had a issue with my fans a while back. The connectors do not have a catch tab on them and will wiggle loose but not completely disengage. That will compromise the amperage to the fan. It will slow the fan, eventually overheat and damage. You will eventually start popping fuses. Make sure both fans are running strong. Champion sells replacements at a reasonable price.
The other idea, is the stat in correct orientation, and have you tried switching back to the original just to be sure the new stat is not causing the issue?