Cooling help?

Problems from Idle to 30mph point to not enough air-flow through the rad.This usually points to the fan. But can include the shroud, a faulty fan-clutch,and airflow to,through,and around, the rad and also out from underneath the hood.
The stat sets the minimum running temperature. If/when over-cooling is possible,the stat will close, to try to keep the engine at a minimum running temperature.
Good cooling from cruising speed, down to about 30mph, indicates that the system is able to shed heat with ram-air flow through the rad, and adequate coolant circulation.