Correct, however my old truck would idle fine at red lights. If the truck was idling for more then 5 minutes even after driving on the freeway, the erratic idle would start back up -not as bad, but it will present itself again.
I'm open to understanding what you are saying but please clarify your position on how a cold start motor with a partially closed T-stat has any impact on the ECU once the system warms up to normal operating temps and the T-stat is fully open?
This is not relevant to TJF's issue at this point because he has already replaced the T-stat and the problem still exists. But good for clearing up any misunderstandings for other readers with similar issues.