I'm not new to these trucks and have read through Extinct's threads many times..

backstory - Have 600 miles roughly on a "new" used 4.6 (unknown history) that I swapped into my wife's green D2. Truck ran super minty for first 500 miles but last few trips idle temps have been creeping up once truck is warm. Truck has all new hoses, WP, radiator, new fan clutch hayden 2781(as of last night and todays drive was no improvement). I did all new gaskets on motor, swapped over the crower cam and lifters from the motor I pulled out.. Been running unpressurized on the inline mod. The only thing I did not replace in this rebuild/swap was the t-stat.

Question for the class is can a failing T-stat cause the issue of idle temps increasing (never let it go over 200) yet drop down to the appropriate 180 or lower once moving?